Hold the dumbbell above you like you're about to do a normal pullover.
As you let your shoulders go into 180 degrees of flexion, flex your elbows and lower the dumbbells right behind your head. The triceps will be in a fully stretched position.
As you come up, extend the elbows and bring the dumbbell back above you.
https://www.t-nation.com/videos/tip-the-pjr-pullover