Holding a plank position, bring your right knee toward the outside of your right elbow using the obliques. Look to your knee as it pulls forward to more deeply engage the abs.
Step the foot back, and do a push-up.
Repeat on the left side.
Continue, alternating sides for 60 seconds.
For an advanced version, you can pull the knee toward the shoulder instead of the elbow. For a modified version, pike the hips back instead of the push-up between spider planks.