Start in a quadruped position. Hands under the shoulders, knees under the hips. Maintain a neutral spine throughout, and keep the hips and shoulders level.
Extend the right hand in front of you, arm straight, while simultaneously extending the left leg behind.
Pause at the top, left hip full extended. Actively reaching your leg behind, and your arm in front.
Return to the start, switch sides and repeat for repetitions.
Keep a stable position throughout, avoid arching the back.
To regress, start by isolating the leg movement.