Originally Posted by
Bob Viking
Stretching your legs out avoids a gap between your thigh and the seat. Otherwise the seat would accelerate into your thigh and snap your femur. Stretch out your legs and let the acceleration and leg restraints pull your lower legs in as you move up the rail.
Additionally, if you push hard against the rudder pedals you push your lower back hard into the seat. This improves your back/spine posture for the ejection.