The Double Leg Pull aka the Double Leg Stretch in the Pilates ab series is one of my favorite. And, it's also probably THE Pilates exercise that preps you for ALL Pilates exercises. It's also one of the easiest to cheat on so be aware of some of the tips I give below. However, if you can really hone in on having the longest length in your spine in both the set up and action of this exercise you are truly going to feel the strength of your center and you will see and feel your entire practice reflect this strength.
Keep the longest length in your spine. Reach your arms from your back
There are two ways to get started in the Double Leg Stretch. You can get into it from laying on your back, bringing your legs into your chest, and your hands can grab your ankles as you curl your head and chest up. Or, from the Single Leg Stretch bend both knees in, and remain curled up. Your heels should be together, your knees and toes are apart and double-check that your shoulders are on your back and out of your ears. You want to feel like you are pulling on your ankles from your back and not from your chest or biceps.
From here, reach your straight arms up and back and your legs out to the high diagonal. Hold here for a moment, double-check your shoulders are still off the mat, your legs are hugging together and you didn't lose your center. Then circle your arms around and bring your knees in and grab your ankles, pull your knees into your chest two times and repeat. Do this again 5-10 times. You are trying to keep the longest length in your spine. As your legs reach away pull your stomach in and curl up and over yourself. Reach your arms long from your back. You may have to tell yourself to pull your shoulders onto your back as they reach away to keep them out of your ears.
The more your seat works in this exercise the more you will also feel your legs are connected to your center and you won't feel like your back is arching off the mat or your hip flexors are overworking.