Come into a plank position with your hands underneath your shoulders and your feet a little wider than hip-distance apart. Engage your core and lower-body muscles by pulling your belly button into your spine and contracting your glutes and quads.
Lower down into a push-up, bending your arms back on the diagonal.
Press back up to the plank position, then lift your right arm off the ground as you simultaneously rotate your body to the left, opening up to the right, and reaching your right arm straight up to the sky. You should be in a left side plank.
From this position, lower your hips toward the ground, then squeeze your obliques (your side abdominal muscles) to lift your hips back up.
From here, rotate your body toward the right and place your right hand on the ground. You should be back in your high plank position.