From what I've heard about in regards to playing high (in terms of aperture) is to make the hole smaller like a pinhole or like a camera lens, but how? Like without having the typical Pucker up embouchure, but more like a "Say Mmmmm" type of embouchure. How does one make the aperture smaller when the aperture looks more like a slit... Also, as I play higher my jaw comes in, making all air go downwards. Does anyone have any advice on preventing that? I've heard its quite a common problem, funnily enough I thought that'd never happen to me when I was younger... Thanks!