As you probably know the Triceps make up the most bulk of your upper arm and have three heads, the Lateral,Medial and Long. The Long Head gets its name because instead of originating off the radial sulcus like the other two it crosses the scapula.
This subtle difference means when you pre stretch the long head by raising your arm you make it more key in extension of the arm and thus emphasizing it.
In the late 50s guys like Reg and Bill Pearl were discovering this and both absolutely loved overhead tricep work as pictured and credited their large triceps to it.
Bottom line if you want thick and shapely triceps these are an incredible tool . You can do them with a barbell , cable or dumbbells in a dozen different positions. Just be sure your upper arm is head level.
Overhead Dumbbell Extension– Keep your elbow in the same starting position throughout the exercise, contract your tricep hard at the top and touch your forearm to bicep (by contracting) at the bottom if possible . You can do these with one dumbbell with both hands or hit each arm independently.
If you notice Pearl is using a barbell, this was also favorite of Parks. Just be sure the elbows stay where they start and you get as full of a range of motion as possible.