Tip: You can use a plate dumbbell, medicine ball or kettlebell for this exercise, or simplify it with no weights.
Start by lying on the centre of your back on a Swiss Ball holding a dumbbell with both arms straight up over your chest.
Rotate the body to one side, twisting at the torso, keeping the arm straight but allowing the arms to also rotate. Return to start position, repeating on the other side. This is one rep.