If you take a Body Concert Hall breath and release that passively, that’s Passive air.
Basically, you’re not using your mouth or abdomen to compress the air into the horn, you’re just releasing the air out in a constant flow.
This is helpful, crucial I think, in separating Shape and Air and learning how Shape changes pitch, independent of Air–which is needed for volume and long notes/phrases.
One of our tendencies is to use our Air as a crutch to kick and manipulate to overcome inefficiencies in our Shape (embouchure, tongue arch, etc.).
By releasing Passive air and focusing on changing Shape and experimenting / observing how changing Shape changes pitch without changing Air, that observation helps us realize we can change pitch independent of Air–which is needed for volume and long notes, not to change pitch.
Less air is required the higher we ascend, which is opposite of what we tend to think. It takes less air to play a High C than a C on the staff.
Playing notes above the staff softer actually makes it easier, especially if you successfully relax, open your throat and avoid clamping your lips. Playing notes above the staff loudly does require Active air / support. But obtaining those pitches is easier than many of us (myself especially) realize initially; we make it more difficult by over-blowing and over-tightening our lips, throat, etc. We fight against ourselves.
Once we find / learn the Shape required to obtain the pitch, we can work on exercises to crescendo / decrescendo to increase our dynamic control, articulation, etc. And learn how to use Active air / support to play louder, longer, etc.
I don’t think it’s really until above High C that air support really gets to be critical; I could be wrong, but I believe that’s the case. I can play High C and D and maybe even E pretty softly / without thinking much about air. I spent a long time on avoiding any sort of air support, just using passively released air to really focus / isolate how Shape effects / obtains pitch, as I didn’t trust myself at first to be able to use Active and Passive air back and forth. I don’t have any playing commitments, so that was easy for me to do.
Hope that helps, my $.02 FWIW.