This exercise works the hamstring and the glutes.
Lie down into supine position onto the ground, place your heels into the handles of the G-Flex then lift your hip and pull your legs under yourself. Then extend your legs to the starting position. Keep your hip high, your shoulder blades shall always be on the ground. Pay attention that your back and torso should be straight all the time. You may place your arms next to your body in the ground but if you are more experienced then fold your hands in front of you on your chest or you can even lift them up.