Stand on the BOSU and balance in the centre.
Shift your weight to one side and lift the other leg slightly. Whilst keeping your back flat and head neutral, hinge at the hips and keep your leaning leg planted on the ground with a slight bend in the knee. As you hinge, raise your other leg behind you so your back and leg become parallel to the ground. Try get this leg straight and keep your arms neutral. As you hinge back to the starting position, follow through with a knee drive and pause for a second. Repeat.