This is an isometric hold. Begin by pulling the belly button into the spine to preactivate the core, then plant your elbows and hold. Try to make a straight line with your shoulders, hips, and ankles.

Lying on your back with your feet off the ground, knees bent at 90 degrees and arms reaching for the sky, slowly and with control lower your left leg straightening it as you do so - as you lower your right arm. Return to your starting position and alternate sides. Try not to move the non-moving leg at all. Toward the end of the video you see me trying real hard not to move that leg but it's hard. Do your best!