Finish: From the basic plank position, rotate your torso outwards to the left into a side-bridge, resting your body weight on your right arm and opening your left arm straight up reaching to the ceiling. Allow your legs to cross as your body turns, top leg in front of the lower leg. Then rotate inwards to return to the plank, and rotate out to the other side. Repeat, alternating sides.
Safety tip: Make sure that you are strong enough to maintain a perfect plank position for at least 30 seconds, and that you are able to maintain good form doing a basic plank and moving your arms and legs in different variations of this exercise, before you progress to this move. If you feel any strain in your lower back or shoulders, choose an easier variation to help build up more strength.