Hits the long head of your triceps for full horseshoe pop.
Only your forearms move, keep your upper arms locked in place.
Taking a single dumbbell overhead puts the long head of the triceps, the biggest of the three, into a deep stretch behind your head, and training a muscle at long length is where the growth stimulus is richest. With both hands cupping one bell and upper arms locked beside your ears, only the elbows move, keeping every rep on the triceps. It is the stretch-position counterpart to pushdowns, and most arms need it more.
The long head crosses the shoulder joint, so it only gets fully lengthened when your arm is overhead. Pushdowns never take it there, this movement does, and that deep stretch behind your head is the whole point.
Both work. Seated with back support makes it harder to cheat with your torso, while standing demands more trunk control. Pick whichever lets you keep your elbows tucked and ribs down.
Some flare is natural, but wide-swinging elbows shift stress off the triceps and into the shoulders. Keep them pointing roughly forward, close to your head, and use a weight that allows it.
