Sit with your legs spread to the sides and do a few sit ups. The purpose behind this is to keep the body straight even if the legs are making the hips less mobile (the same way when you slide forward or to the side) when you move.
Sit with one leg over the other and switch the upper leg all the time and do a few sit ups. This teaches you to feel your balance and tension all the time and to adjust accordingly.
Sit with your legs in the air and do a few sit ups while they are in the air (a bit) this teaches you to move even if you are tensing and balancing at the same time.
The idea is to be aware and to control the tension. Not to relax completely until you die.
