Keep your body as still as possible moving only your forearms.
This exercise can obviously be dangerous when using heavy weight. Start with light weight until you have perfected the movement.
Instructions
Set up for the exercise by choosing a dumbbell and setting it at the end of an incline bench.
Pick the dumbbell off the floor with both hands securing it on one end with your hands flat and palms facing upward.
Slowly lie back on the bench and raise the dumbbell above your head by fully extending both arms. Now you should be in position to start the set.
Slowly lower the dumbbell behind your head by bending only at the elbows. You will be moving in a semi-circular motion until the dumbbell is about even with the top of the bench.
Pause, and then raise the dumbbell back to the starting position.