Obscure Elbows are those strikes you execute when you cannot see the attacker. The Obscure Vertical Elbow is rear elbow strike executed when someone is behind you. Imagine a rear bear hug where you were able to escape but he still behind you trying something else, maybe a choke, then your Obscure Vertical Elbow is perfect for preventing him from completing the choke.
You can see the application of the Obscure Vertical Elbow on this self-defense technique Obscure Wing. Where the attacker grabs the shoulder from behind, you can see the attacker but you know he is in range for your moves. See how GM Jesus Flores demonstrates this technique.
Here you can see GM Jesus Flores executing Obscure Wing a self-defense technique where you cannot see the attacker but you know he is there behind you in a range that you can control.