I was told by a friend of mine the other day that studies with a guy in the Milwaukee Orchestra that when his teacher plays he tries to keep the oral cavity small. So I asked him, "Does he mean that the oral cavity should be the same size (or feel) of that when he is lip trilling or lip slurring?" He said yes.
I'm wondering if this is true because I sometimes have trouble tonguing or slurring after playing really big open-blowing passages. I feel as though my mouth is too open which causes me to move more in order to facitlitate from that open position.