Lie with your knees bent and your arms and heels on the floor. Push down through your heels and squeeze your glutes to raise your body into a straight line from knees to shoulders. Next, bring a knee toward your chest. Reverse the move, then repeat with your other leg. That's one rep. Don't allow your hips to sag at any time during the movement.

From http://asp.elitefts.com/qa/default.asp?qid=4102&tid=102 :
This is one of the most popular max effort squat exercises at Westside Barbell Club. This exercise is performed in one way or another 70% of all max effort workouts. This is because it works the posterior chain like no other exercise. Done properly, this exercise will work everything between your traps to your calves. Begin this exercise by unracking a barbell the same as you would a squat. Set up so your feet so they are slightly wider than shoulder width. Get into a tight position (arched back, shoulder blades pulled together, knees slightly bent, abdominal pushed out against your belt).

Lie face down on the floor with your head resting gently on the backs of your hands. Pull your abs in toward your spine and raise your legs 3 inches off the floor (keep them straight no bending at the knee). Rotate your hips so that your toes turn out, and quickly open and close your legs 6 to 8 inches, keeping them straight. Vigorously beat your heels together for two sets of 30 repetitions.

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