In the showcase outside of Reinhardt's studio was a mock cover of Time Magazine. It was a characature of Doc and captioned "Don't drop the jaw, man!" From second line G on the trumpet (4th line F for trombone)descending, the tendency to drop the jaw (widen the space between the teeth/open the mouth/open the lips)is most prevalent in beginners and thus carried on into the player's ensuing years. In many cases, the lack of a high register can be traced to just such a practice.
In the book The Pivot System Manual, Group II, exercise III Doc states "Descend by relaxation plus the pivot, rather than opening the lip formation". The exercises takes a middle Bb and descends to a low Bb and likewise all 7 finger or slide combinations down to the low E (concert pitches). Without going into the "pivot" which would take an all new post to explain, let's just come up with an exercise that will reduce or eliminate the jaw drop to descend. This is a "tried and true" way to REDUCE OR ELIMINATE the jaw drop without knowing or using the pivot.
Play a middle Bb Concert, normal attack, and sustain for 4 beats. On the same breath, slur down to a low Bb Concert and SIMULTANEOUSLY puff the cheeks out to their fullest on the low Bb and sustain with fermata. [Be careful NOT to reduce the mouthpiece weight on the lips when executing this!] Repeat and rest.
Repeat above starting on the A down to the low A.
Repeat all using all 7 valve (slide) combiniations.
The low register will then respond fully without the need to open the lips, drop the jaw, or separate the teeth.
It really works!
Since we all are teachers, whether ultimately "our own teacher", teaching others privately or teaching the youth in a school setting such as Middle School, High School or College level, I will be gearing FixYourBrass to include teachers and performers. My posts will have various "topics" (Range, for example), with information for development and maintaining these categories as well as problems that may arise. The posts may be followed by my own "comments" and as to solutions to these problems watch for follow-up posts. So please add your own comments as to create a dialogue regarding said posts as well as any "personal problems" that I can help with. Looking forward to this exchange.