Originally in Joseph Pilates book Return to Life, you will see the Single Leg Pull aka Single Leg Stretch and the Double Leg Pull aka the Double Leg Stretch. These were the only two in his ab series. This exercise is a great chance to challenge your center as you switch your legs and to open your hip as you continue to dive into your mat practice. The Single Leg Stretch will prepare you for the Double Leg Stretch which comes next in the order.
Press your leg into your hands, pull your leg from your back!
You can transition into the Single Leg Stretch from your Rolling Like a Ball exercise. Place your right hand on your right ankle and your left hand on your right knee. Stretch the "free" leg out to the high diagonal and your shin into your hands. Begin to roll your back to the mat until you reach the tips of your shoulder blades. This is where you will begin. Or, you can simply begin from lying on the mat grab your right ankle with your right hand and your right knee with your left hand curl your head and shoulders off the mat and reach the other leg out to the high diagonal.
From your start position pull on your shin 2x from your back, not your biceps and then switch your legs. Your hands will also switch. The left hand left ankle and right hand left knee. Again, pull 2x from your back and switch your legs and hands.
The trick is to not let go of your center as you switch your legs. Something your body will want to do! Also, the hand placement helps maintain your shin staying along with your midline and not rotating to the side avoiding the stretch.
If you are new to this exercise, do 5-10 on each side. But, as you advance through your practice take yourself down to 3-5 reps each side and then flow right into the next exercise without taking a break to rest your head.