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

[adwb]

I've been playing around on my K-x and haven't been able to notice the green button switching my ISO mode to AUTO. On the K-x the OK button is not the ISO button. I have tried pressing the ISO button with the green button, but nothing happens. I have tried this in M, Av, and TV modes - all to no avail.

Please read your manual, the above applies to the K10-K20 not sure about others.
the green button options on the K-x and K-r are not the same.

[vincechu]

thanks, everythin seems to work on the K5 too :-)

[bjake]

What green button setting in the menu do you use for 'k' style lens in a k-x?
Jake

[Pepe Guitarra]

Optical Depth of Field

[jcb3210]

谢谢！辛苦了。

[cleveh]

Great summary. Thank you.

[unfocused]

On the *istDS there's no 'green button'. Use the AE-L button which does the stopdown metering and automatically sets the shutter speed.

[TomBrooklyn]

In manual mode 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).
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)

What does "the program mode" mean?

What does "the program line" mean? Could you elaborate? You wrote "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)."

What camera menus make up this "program line?

What does MTF stand for? You wrote "MTF (where the camera sets the lens at the best aperture for that lens, which is programmed into the lens by the manufacturer)." Could you elaborate for those, who like myself, are unfamiliar with lenses that have computer programs in them.

Step Down Metering
With an M or M42 lens, you can use the button to stop the lens down and take a meter reading.

I don't own a Pentax body, but I own a few M42 lenses (that I currently use by adapting them to a Canon body.) Could you explain how pressing the Green button makes M42 lenses change aperture? Is there a mechanical connection between Pentax camera bodies and these lenses that controls aperture?

With a modern digital Pentax, and particularly the K-5, if an M42 mount Takumar switch is set to Auto, will the lens stop down to the aperture set on the aperture ring when the shutter release is tripped?

[Marc Sabatella]

Program mdode is "P" mode, where he camera decides on both aperture and shutter speed for you, based on a variety of different possible algorithms that some cameras let you select between. MTF is something you can Google for moe specifics, but basically it means, sharpness. A camera in P mode might choose an aperture based on what is sharpest, or based on what gives you the fastest shutter speed, or largest DOF.

None of this is relevant for M42 lenses though - only ones in which the camera can control the aperture.

[nhausman]

this is a great source of information!

[gmachine]

New user, nice to know how to use older lens in manuel mode.

[Pepe Guitarra]

... Could you explain how pressing the Green button makes M42 lenses change aperture? Is there a mechanical connection between Pentax camera bodies and these lenses that controls aperture?

With a modern digital Pentax, and particularly the K-5, if an M42 mount Takumar switch is set to Auto, will the lens stop down to the aperture set on the aperture ring when the shutter release is tripped?

I personally have used the M42 lenses in AV ONLY. I have tried to do that in manual, but the camera reverts itself to AV priority. There is no connection between the camera and the lens (that I see). That is the reason the AV priority. You set your ISO first, then you set your aperture, and the camera sets the exposure time for you. If you can focus with the apertured stopped down, good for you. I personally and in most cases have to focus wide open, and then stop down by counting the click to the aperture I want. With bayonet lenses, there is an aperture lever that is controlled by the camera, hence, you can push a green button (it is a button to which you can assign functions, in my case, I assigned the depth of field function. To determine the depth of field, the camera has to stop down and it measures the light in the process.)

On muy K5, if I set the aperture of an M42 lens to AUTO, it will not work. Simply, because the auto function was designed for older film cameras that had a mechanical connection that pushed the pin that comes with the AUTO M42 lenses. The K5 or andy K DSLR cannot push the pin. Hence, the AV priority is the only way to go with the M42 on a K5.

If there is someone that think I am wrong, I welcome it. Because I have read many times that some people can shoot in MANUAL mode the M42 lenses, I have not been able to.

Further more, you will find M42 lenses that are AUTO only (no Man-Auto switch). In order for those to work, you have to keep the auto pin pushed all the way. Some people remove it, I push it and glue it with crazy glue to make them permanently manual, such as the Fujinon 55mm f1.8 lens. A great lens but has to have this surgery.

[Pepe Guitarra]

I have a question of my own regarding stopping down. On the K5, I can also stop down (and meter the light) by pressing the ring around the ON switch from, off, to on, to beyond. This method also stops down the lens and allow you to shoot manual mode lenses. My question is, I have problem with some lenses, this method is not consistent with me, do you have the same problem. The green button works fine, but it is placed in such a weird place that I sometimes lose focus trying to push it.

[riff]

some people can shoot in MANUAL mode the M42 lenses, I have not been able to.

What is the problem Pepe? I can shoot M42 (I have only Taks) in manual with the K5. Regardless of mode you get great results.

[Paul Pheiffer]

On muy K5, if I set the aperture of an M42 lens to AUTO, it will not work. Simply, because the auto function was designed for older film cameras that had a mechanical connection that pushed the pin that comes with the AUTO M42 lenses. The K5 or andy K DSLR cannot push the pin. Hence, the AV priority is the only way to go with the M42 on a K5.

If there is someone that think I am wrong, I welcome it. Because I have read many times that some people can shoot in MANUAL mode the M42 lenses, I have not been able to.

Hi Pepe, Take a look at the info here at this link:
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html

Read through all of it - I hope that will help a bit.