Start in a traditional plank — shoulders over hands and weight on your toes — with sliders underneath the balls of your feet.
With your core engaged, bring your right knee forward to the inside of your right elbow, then slide your right foot under your body, and straighten the right leg. Your pelvis will twist a bit to the left, too.
Bring your right leg back to a tucked position at your chest, and return to your basic plank to complete one rep.