Holding a dumbbell in both hands, sit on the ground with your knees bent and your heels off the floor.
Lean slightly back without rounding your spine at all. It is really important, and difficult, to keep your back straight.
Pull your navel to your spine, and twist slowly to the left, bringing the weight to your left side. The movement is not large and comes from the ribs rotating, not from your arms swinging. Inhale through centre, and rotate to the right. This completes one rep.