This exercise will help stretch the low back and hips. It will help you learn separation of the upper and lower body that is so important for the golf swing.

Lie on your back with your legs in table top.
Stretch your arms out to the side to stabilize your upper body.
Keeping your upper body stable, simply lower your knees to one side, keeping your shoulders pressed to the ground. Drop your knees as far as you can.
Hold the position for a moment and then twist to the opposite side.
Repeat twisting the lower body from side to side for 8 to 10 repetitions.

If you have any restrictions in spinal rotation in your golf swing, you will end up over using other parts of your body like your shoulders and arms causing swing compensations and swing faults.