Most of us brassplayers, when starting to play, tend to form a wrong embouchure, pinching the lips to get a sound. Only a few lucky one (Hardenberger/Helseth etc) who get to a good teacher at the start, avoid this.
Recently I attended a clinic with Wayne Bergeron, where he demonstrated this corners topic. Now WB uses the AAA-OOO setting that Greg show us and explain again and again. WB also showed that this forward setting also helps to tolerate a bit more pressure (sometimes there is no other choice than resorting to that).

Being myself now mostly a french horn player, I have also struggled with this tendency (to pinch when playing high notes). By doing the WindWorks exercises, I have now really “found it”.
My horn teacher was surprised at my last lesson with her on wednesday – how good I sounded and how effortless I played.

Btw, I like Charlie Porters demonstration and explanation on YT (search his name and “embouchure”).