Get into a plank position, making sure to distribute your weight evenly between your hands and your toes.
Check your form—your hands should be about shoulder-width apart, back flat, abs engaged, and head in alignment.
Pull your right knee into your chest as far as you can.
Then switch, pulling that knee out and bringing the other knee in.
Keeping your hips down, run your knees in and out as far and as fast as you can. Alternate inhaling and exhaling with each leg change.