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.