Bring yourself to the ground in forearm plank position with your elbows directly under your shoulders and your legs extended directly behind you. You should have an unbroken line from your head to your toes. Keep your core braced and do not let your hips drop. This is your start position. Proceed to rotate your left hip towards the sky reaching your arm up overhead as you do so. Return to the start position then perform the same movement on the opposite side. Repeat the same sequence, alternating sides each rep, for the required number of reps (or time).