Its important to keep your elbows in and facing toward your hips. Dont let them flare out as you lower the dumbbells down.
Keep the rep timing slow and control the weight throughout the set.
Focus on moving the weight with your triceps and only moving your forearms. Keep the rest of your body as still as possible.
This movement has the potential to be very dangerous. Start by using a very light weight until you have perfected the form.
Instructions
Choose a dumbbell and sit on the end of a flat bench with the dumbbell resting on your thighs.
Hold the dumbbell with a pronated grip (palm facing your feet) with your left hand and lie back on the bench, lifting the dumbbell above your head until your left arm is fully extended.
You can brace your right arm by placing your right hand on the back your left bicep or on your left shoulder.
Bending at the elbow only, slowly lower the dumbbell down beside the right side of your head your head until it is about level with your right ear.
Pause and squeeze the tricep. Then raise the dumbbell back to the starting position.
Do not lock your elbow out and then repeat for desired reps.