This is a great mobility exercise for the spine, providing a great stretch through the upper - mid back and posterior shoulders.
To perform the exercise start by kneeling on all fours, with your hands directly under your shoulders and knees directly under your hips. Lift your left arm and reach up towards the ceiling by rotating to the left. Hold briefly and then reach under and behind your right arm, bending your right elbow and dropping your left shoulder towards the floor. Hold for a count of 3 and then return to the start. Repeat both sides.