This exercise is called a drop-knee, and it is based on mixed martial arts, or MMA, movements. It comes from trainer Jackie Warner, author of the recent book "10 Pounds in 10 Days." It's a very challenging move because it requires that you let your back drop. Warner says to always concentrate on form and breathe through the movements.
What it does
The drop-knee works your core.
How to do it
Position yourself with hands and feet on the ground in a plank position. Your arms should be a little wider than your shoulders, your body straight and your legs spaced apart the same distance as your arms.
Keeping your arms straight, put your weight on one leg and move the other to pass it underneath your body, dropping and twisting that side of your body as you do so, bringing the leg so that the knee and ankle touch the ground. Pull that leg back under your body and place your foot back on the ground. Shift your weight to the other leg and repeat on that side.
Try three sets of 15 repetitions per set.