The following are copy from several different user's post.
Hopefully this can help more newcomers (or not) to know more about the great Green Button.
Thanks to Peter Zack, RuiC, sezme.

In manual mode (A,FA, DA etc series lens) you can push the button and it sets the exposure for the program mode and the program line. Program line is the setting you have selected in the camera menus, like MTF (where the camera sets the lens at the best aperture for that lens, which is programmed into the lens by the manufacturer). Then you can fine tune the exposure to your liking from this setting. Very old school but an absolute requirement for a serious shooter in tough lighting and many other situations. (Peter Zack)

Step Down Metering
With an M or M42 lens, you can use the button to stop the lens down and take a meter reading. For those that own these lenses, it's an approximate reading and you have to understand (with practice) the way your particular lens will meter on the attached body. In most cases, these lenses meter well wide open but tend to over expose in middle apertures. So you use the green setting recommendations as a starting place to adjust to the correct exposure. (Peter Zack)

Return to HyperProgram(P) after changes
In HyperProgram (P) mode you can turn the front e-dial to adjust exposure time other than P suggests and it changes to Tv mode. If you turn the rear e-dial it becomes Av mode. Then press Green-button it returns to P mode again. (RuiC)
image from Digiichi - Next generation hyperprogram.

From Full Manual mode to P, Av or Tv
The green button in M mode get you to P, Av or Tv dependant on what mode is chosen in the green button menu set up. (excellent if you want to use it like an on-demand spot-meter)

The green button doesn't change from M (manual) mode to any other mode - it leaves you in M mode. It just momentarily gives you the effect of being in one of those other modes (which one depends on how you've set it up in your custom options) - setting the same aperture and/or shutter speed that you'd get it you were in one of those other modes, but then leaving that set. As opposed to constantly readjusting as the light changes.

(thanks lol101, Marc Sabatella, adwb)

Act as Reset button: Reset Exposure Compensation, Flash Compensation
If you have set exposure compensation, hold the exposure compensation button plus the green button will reset the meter to zero E.C.
* You can also reset the Flash Compensation with green button. (thanks rockmaster1964)

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!
Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO. (thanks lol101)

Zoom in in Playback mode
In playback, after you zoom, Green button zooms IN and EV+- zooms OUT

[sebberry]

Care to expand that? Maybe we can offer some advice.

Excellent post vizjerei

I guess it has to do with my more P&S style of using the camera.

I have a decent understanding of how Av, Tv and TAv modes work, but I have no clue what P mode does.

My limited knowledge of the camera usually flies out the window when I take a photo, but I do find the camera controls themselves very easy and intuitave to use, even if I don't really know how they affect the picture.

[Canada_Rockies]

I guess it has to do with my more P&S style of using the camera.

I have a decent understanding of how Av, Tv and TAv modes work, but I have no clue what P mode does.

My limited knowledge of the camera usually flies out the window when I take a photo, but I do find the camera controls themselves very easy and intuitave to use, even if I don't really know how they affect the picture.

In P (program) mode, the camera sets everything: Shutter speed, aperture and ISO (within the limits you have set). It is different from the green setting on the mode dial in that you can use EV Comp and a few other things that are locked out when in full green mode.

[Peter Zack]

To expand CR's comments on the P mode. A lot of people here don’t use the P mode, but I like it. Most will shoot in Av or Tv modes. P mode is 3 modes in one. Turn it on and hit the green button (unique to Pentax) and the camera sets the controls according to the program line you set in the menu (MTF (best quality from the lens as programmed into the lens chip by the manufacturer, Av priority or Tv priority). If you don’t want the camera to do that, you can then turn the Sv e-dial to adjust the shutter speed to your needs (the camera goes into Sv mode). Or you could adjust the aperture with the other dial and the camera goes into Av mode. To reset the camera, hit the green button and let the camera make the choice.

To me it's a great setting.

[Leaf Fan]

Wow, thanks everybody for that. I had no idea the green button could do so much. All I've been using it for is to reset the auto settings in P mode after I've made an adjustment to the aperture or speed.

[Gulam]

Fantastic Post - I have a K20D and I know what the green button is......

[ezra]

Groovy, thanks for that. Didn't know you could quick change the ISO like that.

And here I was going through the Fn button for half a year. Great post, thanks Vizjerei!

[hlpentax]

I just bought a nikon d700 and miss the pentax green button very much. Anyone know if Nikon has an equivalent function?

[Peter Zack]

Yes they do. the way it works is this. First you... hold on a sec the phone is ringing....

[rayallen]

To expand CR's comments on the P mode. A lot of people here don’t use the P mode, but I like it. Most will shoot in Av or Tv modes. P mode is 3 modes in one. Turn it on and hit the green button (unique to Pentax) and the camera sets the controls according to the program line you set in the menu (MTF (best quality from the lens as programmed into the lens chip by the manufacturer, Av priority or Tv priority). If you don’t want the camera to do that, you can then turn the Sv e-dial to adjust the shutter speed to your needs (the camera goes into Sv mode). Or you could adjust the aperture with the other dial and the camera goes into Av mode. To reset the camera, hit the green button and let the camera make the choice.

To me it's a great setting.

Peter,
You took the words right out of my mouth. But you probably explained it better than I could have. I totally agree with you. After a lot of testing I find that P mode is now my preferred mode for the reasons you have stated.

And I just love all the things that the GREEN button can do.

[soulman]

so far, i've only used it while my M lenses attached to have an automatic shutter speed, but i know it should have more use. thanks for the post..

[MSM]

Great thread everyone. That is what I love about this site. Always something new. Peter, nice description of the P mode. I have always been an Av user. I should try the P mode more.

[filmamigo]

I just bought a nikon d700 and miss the pentax green button very much. Anyone know if Nikon has an equivalent function?

Peter Zack, your response made me laugh!

The "green button" is the extra ingredient that makes HyperProgram really sing.

Nikon has a feature identical to HyperProgram -- they call it "Flexible Program", or generically "program shift." But without a green button to reset the Flexible Program settings, it is less useful than Pentax's implementation. When I shot Nikon, I wished for something like a green button reset... when I switched to Pentax, there it was!

To quote the Nikonian's guide to the F5 (most of the modern Nikons are the same):

If you adjust the default pair [aperture/shutter] by rotating the MCD [control wheels] in this mode, you will see a small star (*) to the right of the P in the top LC-Display. Nikon calls this the "Flexible Program". The flexible program stays in effect until:

a) the metering system turns off (after a couple of seconds. The metering system on-time can be set to 4, 8, 16 or 32 seconds via CS #15. I like to save power, so I have it set at 4 seconds while 16 seconds is the default)
b) you change exposure mode
c) you turn off the camera
d) you reset the camera (by pressing the BKT and CSM buttons simultaneously for at least two seconds)

Quoted from: The Nikon F5

I ended up continually turning my Nikon OFF then ON again.

[Peter Zack]

Oh yeah, I'm off the phone now.

Well I noticed the Pentax hyper program coming to other brands over the years, Pentax was first (again) to introduce a great feature set and still have the better method of doing it.

[jhilla]

The plethora of options for the famous green button are endless. You'd be surprised at the ability to customize the green button. I'm at work and I can't indulge all the options but for those of you with a K-x, turn on your camera, push menu, cycle to the right maybe 4 or 5 pages and you'll notice "Green Button", push the arrow and enjoy!

I like the preview shot option, where the green button takes a preview of how the shot will look like without saving the pic. This is really helpful for rookies like myself, I took several shots of these Howler Monkeys when I was in Mexico. Not until we stopped shooting to look back at our shots and mine were either way too dark or under exposed.

I'm sorry my detail instructions might be a off, I don't have my cam in front of me. I've read my manual back and forth and I wondered what this green button did cause my girlfriend's T1i has no such thing and I mean what good is a button if you don't know what it does.

my two cents

-Jon

[Rees]

The following are copy from several different user's post. Hopefully this can help more newcomers (or not) to know more about the great Green Button. ( I have used my K10D for more than 2 years and still didn't know some of the following, if someone can make this a Sticky Thread, it will be great ) Thanks to Peter Zack, RuiC, sezme.

In manual mode (A,FA, DA etc series lens) you can push the button and it sets the exposure for the program mode and the program line. Program line is the setting you have selected in the camera menus, like MTF (where the camera sets the lens at the best aperture for that lens, which is programmed into the lens by the manufacturer). Then you can fine tune the exposure to your liking from this setting. Very old school but an absolute requirement for a serious shooter in tough lighting and many other situations. (Peter Zack)

Step Down Metering
With an M or M42 lens, you can use the button to stop the lens down and take a meter reading. For those that own these lenses, it's an approximate reading and you have to understand (with practice) the way your particular lens will meter on the attached body. In most cases, these lenses meter well wide open but tend to over expose in middle apertures. So you use the green setting recommendations as a starting place to adjust to the correct exposure. (Peter Zack)

Return to HyperProgram(P) after changes
In HyperProgram (P) mode you can turn the front e-dial to adjust exposure time other than P suggests and it changes to Tv mode. If you turn the rear e-dial it becomes Av mode. Then press Green-button it returns to P mode again. (RuiC)
image from Digiichi - Next generation hyperprogram.

From Full Manual mode to P, Av or Tv
The green button in M mode get you to P, Av or Tv punctually (excellent if you want to use it like an on-demand spot-meter) (thanks lol101)

Act as Reset button: Reset Exposure Compensation, Flash Compensation
If you have set exposure compensation, hold the exposure compensation button plus the green button will reset the meter to zero E.C.
* You can also reset the Flash Compensation with green button. (thanks rockmaster1964)

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!
Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO. (thanks lol101)

Zoom in in Playback mode
In playback, after you zoom, Green button zooms IN and EV+- zooms OUT

Well compiled and informative