Set-up: Begin on all fours and place one hand on the back of your head.
Movement: Keeping your core braced and with minimal movement in the lower back, rotate your upper back downwards and bring the elbow of your elevated arm down towards the elbow of your bracing arm. Slowly reverse the movement and rotate your upperback and arm upwards so that your elbow now points towards the ceiling (or as far as you can comfortably get without rounding or hyper-extending the lower back).
Repetitions: Perform all prescribed reps on one side, then on the other. This counts as one set.