Start at the left edge of your mat. Stand up straight with your feet about hips-width apart.
Quickly drive your right knee up toward your chest, bringing your left arm up at the same time. Engage your abs as the knee comes up.
Bring the right leg back down, moving slightly to the side as you land on the ball of your foot, then drive the left knee and right arm up.
Each time you raise your right knee you'll continue to land out to the side, so that you're moving laterally across your mat.
Shuffle across until the you reach the end of your mat, then switch sides, moving to the left this time. Continue back and forth for the full 40 seconds.