This exercise is a preparation for the upper body to do the Classic Pilates Swimming exercise.
To do this exercise you will need a mat. Start by lying on your belly with your legs long beneath you and your arms reaching long along the floor beyond your head. Exhale as you hollow your abdomen and then inhale as you reach your head away from your tail and then lift your head, neck, and shoulders in spine extension. Then continue breathing as you lift one arm higher and then the other. Continue alternating the arms for 10 cycles, and then exhale as you release your spine to the floor.
Things to think about
Often this exercise is challenging on the shoulders. Feel free to widen the arms if you feel any impingement on your shoulders. Also be sure that you are sliding your shoulder blades away from your hands.
As you are extending your upper spine be sure that the front of your hips are open, and reach your big toes away from your tailbone. All of this will allow you to find the longest possible spine.