Q: I have a client who uses a velar fricative for the sibilants. I was experimenting with some of your self-assessment exercises in your book Frontal Lisp, Lateral Lisp, and I noticed that as I dropped my jaw to produce /s/, the sound eventually became a velar fricative. Do you think jaw grading or stability exercises will help my client with this?

Position of the tongue relative to the palate is directly related to jaw position, height, and stability as you have discovered.  Correct sibilant phonemes only can be produced with the jaw high.

Lowering the jaw pulls the tongue away from the palate, especially the anterior part of the tongue.  When the jaw is very low, the only part of the tongue that can easily reach the palate is the back. Therefore, when the jaw lowers, frication naturally shifts from anterior to posterior.

Stabilize your client’s jaw in a higher position and see what happens. Then go from there.