SS should not put you in a chair seat (thighs high/etc), in ALL seats (by the usef rules...) ear/shoulder/hip/heel should be in alignment, and there should always be a straight line from elbow to horses mouth. Yes, the hips should be more open, which almost makes for a lower leg too stretched/calves almost off. It should NOT be about raising the hands however (there is a straight line but horses are (articially) high), but not resting them too low either. Affectations of style (over doing it are everywhere).
The difference is that one is (almost) behind the motion in ss, with it in dressage, slightly in front of it in h/j (because of moving at speed and over fences).
Least we all forget people like Bill Steinkraus/Wofford/etc rode ss before they rode huntseat/jumpers/evented!
Keep the thumb onto the forefinger to keep a connection and prevent dropping the rein. Erect posture will help prevent stumbling/loss of balance/etc.