Start in a plank position. Swing one leg out and up to meet your foot with your hand. As you return the foot back to its starting position, swing the other foot to meet the other hand. This is considered a more advanced version of the Mountain Climber. Count either side or both as one repetition.
Modified version, to use as stretch or lower impact alternative to regular exercise: Bring leg up in slow, controlled fashion. Hang out in this stance for 3-5 seconds, stretching limbs. Bring leg back to original position, and bring other leg up for the same stretch. Each side counts as one repetition.