how to do the single straight leg stretch

Begin by lying flat on the mat but with your legs extended toward the ceiling. Legs and heels are together in Pilates stance, rotated slightly outward from the hips.
Lengthen your spine, pull in your abdominals, and curl your chin and upper body up off the mat. The tips of the shoulder blades touch the mat. You will maintain this upper body lift throughout the exercise. It is supported by the abdominals, not by straining the back or neck. Chest lift is a good building-block exercise for this move.
Grasp your right ankle, or below the knee if you have tight hamstrings. Stretch your left leg out at a 45-degree angle.
You can adjust the angle of the outstretched leg to make the exercise more or less difficult. The lower the leg, the harder the abdominals have to work to maintain alignment.

ale: Gently pull your right leg toward you. Pulse the leg toward you twice, increasing your stretch.
Switch legs quickly.

Exhale: Pull your left leg toward you. Pulse the leg toward you twice, increasing your stretch.
Switch legs quickly.
Repeat 6 to 10 times.
If you begin to feel a strain in your neck, it is time to rest and then begin again