Start in a traditional plank — shoulders over hands and weight on your toes.
With your core engaged, twist to bring your right knee forward under to your left elbow. Return to your basic plank to complete a rep. Switch legs, bringing the left knee forward to the right knee for your second rep.
Do 20 reps, alternating sides.
Mountain climbers are such an effective exercise, and adding a twist really works the abs from all angles.