This is a great exercise for building mass and strength in the triceps.
It works the entirety of the triceps.
Start with a weight you can handle easily, increasing the weight with each set.
You should be able to add a small amount of weight each week with reps of 5-10. When you can no longer add weight change to a new exercise.
Sit holding a dumbbell in both hands above your head.
Keeping your triceps locked.
Slowly lowering your forearms until the dumbbell is directly behind your head, then extend your forearms up above your head with some force.