Single-Arm Dumbbell Triceps Extension

Trains the Long Head.

How to do it

Stand or sit holding one dumbbell overhead with a straight arm, palm facing forward.
Set the upper arm beside your ear and keep it there.
Rest the free hand on your hip or support the working elbow lightly.
Bend the elbow to lower the dumbbell behind your head.
Go down until you feel the stretch along the back of the upper arm.
Straighten the arm back overhead, then finish the side before swapping.

Letting the elbow flare out to the side. It points forward and stays there; flaring turns the movement into a partial press and shortens the range.
Letting the upper arm drift forward. It stays beside the ear — once it travels forward, the long head is no longer stretched at the shoulder and the reason for going overhead has gone.
Arching the lower back as the weight goes behind the head. Ribs down and stomach braced is what keeps the spine out of it.
Going heavy. One dumbbell behind your head on a bent elbow is the least controllable position in any triceps exercise, and a poor place to find out the load was ambitious.