This is a very difficult exercise that will help to build great abdominal strength and lower body strength. You need to be flexible to perform this exercise.
Begin by taking a barbell from the ground and extending it overhead. There are 2 ways you can do this; snatch the weight, or clean and press the weight. If you have a rack that you can set high enough, you should use this. Once, you have the barbell extended overhead, step back into your squat stance. Now, slowly lower your body until your thighs are parallel to the floor. You should sit back and bend through your knees when you squat down, do not allow your knees to go in front of your toes. Now, return the weight to the starting position by pushing through your heels and driving with your thigh muscles. Repeat this movement until your set is complete.
The barbell should be slightly behind your head. Also, keep your head up and your back straight, so you can keep maintain balance.
Do not perform this exercise unless you are an advanced lifter with a few years of experience.