I find it easier to instead of stretching and turning away from the spot I am trying to shave, I bunch the skin up by twisting towards the spot I am shaving. It is kind of tricky to get the razor into the tight spot that is left, with my head turned towards where I am shaving and my other hand stretching a little, but it makes for a good flat spot to shave. Use a really light touch as the skin is not stretched as taught as it is otherwise but it has never caused me too much trouble. IME it causes far less irritation than trying to shave over the jaw bone, no rigid hard areas, just flat fleshy areas that are easy to get the razor to and keep the correct angle. This works really well on the outer ends of the jaw towards the ear, not so much the closer to your chin you get, but on my jaw the angles are far less pronounced towards the chin than towards my ears and are fairly easy to shave normally traveling over the jawline.

One caveat, I dont do ATG passes so I dont know how the looser skin will react to that pass.