This exercise is used to work the lower abdominal region and will also help to loosen your lower back.
Begin by lying flat on the floor with your hands to each of your sides; place your palms flat on the floor. Now, bend your knees and pull your legs upper legs to the middle of your torso. Make sure that your lower leg remains parallel to the floor throughout this movement. Next, contract your abs and pull your legs all the way to your chest. Slowly return your legs to the starting position. Repeat this movement until you have completed your set.
You can also place your hands under your butt; some people find this easier. Do not use momentum to throw your legs up, control them throughout the movement; use your abs.
To make this exercise harder you can hold a medicine ball or a dumbbell in between your legs. Or you could use exercise bands.