Sit on a mat, your calves are resting on a bench, the knees make a right angle. Hold your hands behind your neck. Go now up with a rolling movement of your back, you should feel how the individual vertebrae lose contact with the mat. At the highest point, contract your abs as much as you can and hold there for 2 sec. Go now down, unrolling your back.

The diagram shows the most used muscles on this exercise