Seated or standing, hold one head of a dumbbell with each hand overhead so the palms face each other. Keeping the elbows as high as possible, bend the arms to lower the weight as far as you can, then lift into full extension, again trying to keep the elbows high instead of driving them forward.
 
Purpose
The hammerhead overhead tricep extension is a simple exercise for all three heads of the tricep. The neutral grip created by holding the ends of the dumbbell keep the elbows more forward, which allows more lateral tricep head involvement than most overhead extension exercises.

Programming
Generally sets of 6-15 are appropriate.