Both hands, one dumbbell, deep overhead triceps stretch.
Keep your elbows tucked in and pointing up, brace your core so you don't arch your back.
Holding one dumbbell in both hands overhead puts the triceps' long head, the biggest part of the muscle, into its deepest stretch as the weight drops behind your head. Extending back to lockout loads the triceps through a range that pushdowns never reach, which is exactly why overhead work is the growth move of the triceps family. All it takes is one dumbbell and the discipline to keep the elbows pointing up.
The overhead position stretches the long head of the triceps fully, and stretched-position loading is where growth comes from. Pushdowns train the same muscle in a shorter range. Both are useful, but if size is the goal, the overhead stretch earns priority.
Stand it on one end, grip the top plate with both palms underneath, then hoist it to your shoulder and press it up. Reversing that on the way down beats dropping it behind your head when your triceps are cooked.
The weight is too heavy for your current stretch strength. Choose a lighter dumbbell, keep the elbows squeezing in toward your head, and lower slower. Flaring turns the extension into a strange pressing movement and unloads the stretch.
