Hi
If you want to vacate matter (such as dust) from a "blind hole" such as the mirror chamber of the camera you do need more air pressure than ate blower can provide. All you do is create is turbulence in a confined space. The bit of dust this weakish puff of air will dislodge will uncontrollably be re-distributed inside the chamber according to the law of chaos. You will have no control over it. By chance some dust may indeed find itself escaping through the lens mount opening and it makes absolutely no difference whether the opening is facing down, to the side or up. The rest, even the majority will be redistributed according to the law of chaos.
Again you need a much stronger stream of air to generate enough back pressure to force air back out of the mirror chamber and taking all the crap with it. But this high pressure would destroy the camera.
Would you put a camera with the lens removed, mount pointing up, on the table for any lengths of time? Of course you would not. Because rightly you would assume that the more or less dust laden ambient air would deposit dust into the opening. Right then, the same "dust laden" air you don't want to expose the up pointing camera to is the same air which is available to your blower. It goes in one end of the blower and comes out the other. You now have a neat little "dust cannon".
And here is another thought: If you could make visible the dust and crap which sticks on the inside wall of the blower the same way as it shows on the sensor you would be absolutely horrified. And this is the device you use to blow air into your cam ?
Would you use a blower to remove dust from the mount end of a lens ? I know you would not, because you know that there is a very real chance that you will blow dust through the gaps that are there and inside the lens.
Would it not be better to aim at dust on the sensor with a statically charged nylon brush ? Pick it up, remove it, dispose of it (not re-locate it). It is safe, precise and easy. And if this does not work because the crap has "welded" itself to the sensor surface a blower won't work anyway. You need a wet clean.
So, using the blower to remove dust from inside the camera is useless, I wont use it to remove dust from the back of lenses (see above) and for the front glass of a lens I use a brush (always have) or a lens cloth, what then am I using the blower for ? To coin an Australian phase: I'll be buggered if I know !
Guys, if blowing inside your camera works for you, or you
think it works, keep blowing. You and I live in a free country.
Greetings
P.S. I know I am never wrong, and everybody is entitled to my opinion.