Help Your Hyoid!
The only free-floating bone in your body (i.e. not attached to your skeleton) is just under your chin, acting as an anchor for the muscles of the front to the throat, the jaw, the tongue, and any support structure of the larynx. Sticking your chin forward pulls on this bone, and thus the larynx, interfering with sound production, especially at higher ranges. So keep that chin relaxed and back!