Keep your body as still as possible moving only your forearms.
This exercise can obviously be dangerous when using heavy weight. Start with a light weight until you have perfected the movement.
Instructions
Set up for the exercise by setting a pair of dumbbells at the end of a decline bench.
Pick up the dumbbells off the floor or have a training partner hand them to you.
Using a neutral grip (palms facing in), position the ends of the dumbbells on your thighs, and lie back on the bench with your feet secured in the padding.
To get in position you raise the dumbbells above you, only slightly bending at the elbows.
Slowly lower the dumbbells down by bending only at the elbows. You will be moving in a semi-circular motion until the dumbbells are about even with your ears.
Pause, and then raise the dumbbells back to the starting position.