Overhead stretch that builds the long head.
Keep your elbows tucked and pointing forward. Don't let them flare out wide.
Pressing one dumbbell overhead with both hands and lowering it behind your head puts the triceps' long head, the biggest of the three, under load at its fullest stretch, since the long head crosses the shoulder and only fully lengthens with the arm overhead. Nothing else moves, just the elbows hinging, so the whole load lands on the target. It earns its place as the single best angle for building the mass of the upper arm's rear side.
The long head of the triceps crosses the shoulder joint, so it only reaches full stretch with the arm overhead. Pushdowns train the triceps in a shortened position, overhead work loads the deep stretch, which is where growth potential is highest.
Both work, and the movement allows either. Seated with back support removes the balance and bracing demand so you can focus purely on the triceps, standing forces the core to hold the position. Beginners usually find seated easier to feel.
Some flare is natural, but wide-swinging elbows shift work off the triceps. Use a lighter dumbbell, squeeze the weight between both palms, and think about keeping your elbows pointing forward past your ears throughout.
