Single-Arm Cable Triceps Extension

Trains the Lateral Head and Medial Head.

How to do it

Set a single handle on a high pulley and take it in one hand with an underhand or neutral grip.
Stand a step back with your elbow tucked against your side.
Start with the forearm at about parallel to the floor.
Push the handle down until the arm is straight, keeping the elbow pinned.
Squeeze briefly at the bottom.
Let the handle rise under control until the forearm is back to parallel, then finish the side before swapping.

Letting the elbow travel forward and back. Once the upper arm moves, the shoulder and the lat are pushing the handle down and the triceps are going along with it.
Bracing the working arm's elbow with the free hand. Holding the elbow feels helpful and it removes the work of keeping it still, which is a real part of the single-arm version.
Turning the torso toward the working side. One arm means the trunk has to stay square against an asymmetric pull, and rotating is how that gets avoided.
Expecting the underhand grip to reach the long head. The elbow is at the side, so the long head is shortened at the shoulder whichever way the hand faces — an overhead extension is the movement that reaches it.