Holding a dumbbell in one hand, lie supine (on your back) on a flat bench.
Raise the dumbbell vertically above your body and hold it using a neutral grip (palm facing out to the side).
Place your free hand just under the elbow of your loaded arm for support.
Keeping your upper arm still, inhale as you flex your elbow and lower the dumbbell to the side of your head in a semicircular motion.
Exhale as you extend your elbow and push the dumbbell back up to the starting position.
Comments and tips
Keep your upper arm still and vertical. Only your forearm should move.
Do not lock out your elbow at the top of the movement.
The lying one-arm dumbbell triceps extension is a unilateral exercise (an exercise with which you train one side of your body at a time). As with all unilateral exercises, start with your weak side, and do not perform more repetitions with your strong side. This will promote equal contralateral strength (the same strength in both sides).
Instead of lowering the dumbbell to the side of your head, you can internally rotate your shoulder and lower the dumbbell across your body to your chest. See the second video.
Of course, you can also perform the lying dumbbell triceps extension bilaterally (using both arms at the same time). See the third video.