Hold a cable handle with a relatively narrow grip overhead facing away from the machine and lean away slightly as needed. Keeping the elbows as high as possible, bend the arms as far as you can, then extend completely, again trying to keep the elbows high instead of driving them forward. Lower again under control.
 
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.
 
Programming
Generally sets of 6-15 are appropriate.