Triceps Pushdown

Trains the Lateral Head and Medial Head.

How to do it

Attach a straight bar to a high cable pulley and stand facing it, about a foot back.
Take an overhand grip on the bar with your hands roughly shoulder-width apart.
Pull the bar down until your upper arms are against your sides and your elbows are bent to about ninety degrees. This is the start position.
Keeping your upper arms pinned to your ribs, straighten your elbows and push the bar down until your arms are fully extended.
Squeeze briefly at the bottom, then let the bar rise back under control until your forearms are just past parallel to the floor.

Letting the elbows travel forward and back. Once the upper arm moves, the lats and shoulders start pressing the weight down and the triceps do less. The upper arms should be as still as if they were strapped to your sides.
Leaning over the bar to use bodyweight. Bending forward turns the movement into a partial press and lets you use a weight your triceps could not move on their own. Stand tall, or brace lightly with a split stance.
Treating pushdowns as complete triceps training. With the upper arms at your sides, the long head of the triceps is shortened at the shoulder and cannot contribute much — and it is the largest of the three heads. Adding an overhead extension, where the arm is raised and the long head is stretched, covers what pushdowns structurally cannot.
Flaring the wrists or letting them bend back. The forearm should stay in line with the back of the hand so the load goes through the triceps rather than the wrist.