Lie on your back on a firm surface. Bend the knees to 90 degrees and firmly hold a soccer ball between the knees.
In an 'unwinding' movement, slowly lift the buttocks and lower back.
Hold this position for a few seconds.
Slowly allow the lower back and then the buttocks to 'unwind' to the resting position.
Relax, and then repeat ten times.
This video demonstrates an exercise to strengthen extension of the trunk.
In case you cannot see the video, here is a still sequence -