W Scott,
I see you refer to PEEP ... the whole back-pressure thing. If you get that feeling when playing particular horns, "that" feeling (of using the the plastic bag) then something is very ... I'd hate to say wrong, but very in-efficient. You're either really fighting a horn that's got some serious problems, or you're grossly using more effort than the set up (mpc / trumpet) requires. Or ... all things being equal, the wrong set up for you.
And ... putting your hand ove the bell of a horn ... no you're not going to feel the air coming out. The horn is an ... amplifier or resonating tube ... for lack of a good term. It's not a fire hose. It takes very little air to make any particular not speek on a trumpet. A little more in the low register than the high register. And yes you'll need to have more air in your body to play a longer passage than a short one. Pop you hand on your mpc !!! vertualy "NO" air is required to make that note speek.
I guess this thread is getting off topic, but that's ok ... it's a great subject to talk about anywhere! It's very a conventional school of thougt to thinking there's lots of air going into a horn to make the notes happen. It's the way most of us are taught the first time we are given that "beast" to play. But, if we start thinking in terms of "efficiency" - "relaxing" - "backing-off" playing can become easier, more enjoyable, the sound gets better, and we put less stress on our bodies. (( of course the medical field will suffer just a bit as there'll be less "$" spent on hernias, strokes, and extreme lower G.I. oriented problems
)) hahaha
Great topic ... let's make playing efficient!