This is interesting. I have always allowed a small amount of air behind my upper - well not lip, but gum area. I've wondered about it, I have had only two private teachers, one local, and he never said anything about it, and my current teacher via web cam. Neither have remarked on it - possibly in the case of my current teacher because he can't see it. It's very slight, and I don't puff my cheeks, and it's only under my upper area above the lip/beneath the nose, whatever that's called. It's not caused me a problem that I have identified, but as I say I've been curious about it. I've been able to play without it, but more comfortable/more secure in the upper register with it. As an example, I just did Smith's flexibilities 1-4 (tongued and slurred), concentrating NOT to use this. All notes were in the accessible to me, but I missed the high C a few times, then tried again. Would not have missed with the air behind there . . . I did (at least I think) notice a difference in my approach, I'll have to look at this more. Is this what you are talking about? And is it definately a problem? Or are we talking about something else/something more?