Hold a cable handle with a one hand overhead facing away from the machine and lean away slightly as needed. Keeping the elbow as high as possible, bend the arm as far as you can, then extend it completely, again trying to keep the elbow high instead of driving it forward.
 
Purpose
The overhead cable tricep extension strengths all three heads of the tricep, but emphasizes the long and medial head over the lateral head. The angle of force is more horizontal than a barbell or dumbbell overhead extension, so the tension is greater as the arm nears extension. Using a single arm develops and preserves symmetry side to side.
 
Programming
Generally sets of 6-15 are appropriate.