This is a modified pullover using a dumbbell where the emphasis is placed on the triceps rather than the lats or pecs.
Begin with the dumbbell over your face and then lower it behind your head. Focus on stretching the triceps. Think about dropping the dumbbell down to the floor, directly behind your head.
Instead of pulling the dumbbell back over your face using your lats or pecs (that will happen to some degree anyway), focus on using your triceps by prioritizing elbow extension instead of shoulder extension.
Think heavy and high reps here: Two sets of 15-20 reps, going hard in the paint on both sets. Trust me, two all-out sets will have your triceps screaming for days.