Start in the push-up position with your back flat and hands and toes squarely on the ground. Bring one knee forward so that your foot is flat on the ground just behind the plane of your hands.
Extend your opposite-side arm straight up perpendicular to the floor, opening up at the hip to turn your upper torso to that side, keeping your core tight. Hold that top position for a count, then bring your hand back to the start position and kick your leg back.
Repeat to the opposite side, alternating sides for the required number of reps.