Because the triceps perform concentric elbow extension, pushdowns from an overhead cable are an effective exercise.
The key is to stabilize the shoulder joint by maintaining the position of the humerus.
The isolated elbow joint performs an arc of motion that allows the hands to move directly below the shoulders.
This position is necessary in order to achieve full extension.
Another key point in achieving full extension is to grip the bar with a shoulder-width grip and a neutral (palms facing each other) hand position.
This requires a long rope or a shoulder-width parallel handled bar, which is most comfortable and efficient.
As with the biceps, creating variation requires a change in shoulder position.
Accomplish this with an overhead position.
Once the shoulder position is chosen, it should be maintained as the arc of the elbow motion is performed.
Although bench dips involve the triceps, they do so at a great risk to the shoulder joint, due to excessive extension of this joint under load.