That's an interesting question, I don't really know the answer! Maybe we can ask Larry
The thing I can say is that every player holds his/her mouth a bit differently, and there can be very subtle effects arising from the 'shape' of the mouth and other parts of the oral cavity. Experimentation with all these different 'shapes' will show you what works best for what 'sounds', and really good players can switch from one 'shape' to another to get the desired effect they are after.
Does that make sense? I guess your question is one variation of the 'cheeks in' or 'cheeks out' issue. Being mindful of how the air is circulating within the oral cavity helps in knowing what movements and 'shapes' produce what effects.
Guan