Stand up straight with the water at chest depth and your feet shoulder width apart
Soften your knees.
Lift one arm out in front of you and the other behind you, keeping your elbows straight.
Tighten your stomach muscles and keep your trunk still.
Quickly switch your arms over, driving against the resistance of the water, so that your arm in front becomes your arm behind and vice versa.
Don’t allow your trunk to twist throughout this exercise.