I posted an article about improper elbow striking. Pictures are located at the following link:
Elbow - Nerve Damage, How Not to Do it! « Muay Thai Techniques
I run a drill with my wife where we "pretend" she's being attacked by a rapist, crazy guy, whatever and she attempts to distract/knock out the individual with an elbow. (I know, I'm nuts, I need help!) Anyhow, I pin her against a wall and I make her fight to keep her hands in the ready position (hands to the face). She then attempts to throw elbows to the temple, chin or orbit (hole where the eyeball sits). I use focus mitts for the drill and I make an attempt to keep her arms down.
Well, we were practicing this drill a while back and instead of striking the focus mitt, she decided to hit me in the temple. Now, I couldn't get mad because he did what she was training to do, but lucky for me she didn't strike me with the tip of her elbow, but with the side of her elbow. She winced in pain, said her fingers fell asleep and she needed a few minutes to recover. This typically doesn't happen when you strike Thai Pads or Focus Mitts, but will happen if you hit something solid like a wall, table or your husbands temple (Ever hit your "funny bone"?). God forbid this happens in a fight in a cage or a fight for you life.
What you'll notice as you train people is that their form deteriorates as they get tired. In the case of the horizontal elbow, the hand should travel across the face towards the opposite ear. When folks start getting tired, you'll notice the hand drop towards the hip and the elbow will strike on the side. The reason you don't want to strike in this fashion is you'll hit your "Funny Bone" or what is better known as the Ulnar Nerve (UN).
The Ulnar Nerve (UN) runs the length of the arm from the shoulder to the hand. You can locate your UN by feeling the tip of your elbow and then moving towards the bend in your arm. If you flex/bend your arm back and forth, you'll feel the "rubbery" tendon. When you hit the Ulnar Nerve, your pinky and ring finger will fall asleep and it hurts like crazy. If you hit your Ulnar Nerve hard enough you can actually cause damage to it.
Here are some pictures of my wife and her girlfriend showing a proper elbow and an elbow that will strike the Ulnar Nerve.
Good elbow strike with the tip of the elbow making contact. The hand is relaxed and comes across the face.
A "not so good elbow" where the side of the elbow is used to strike horizontally. This would be a good elbow if it were used to strike down, kind of like an overhand punch, but since it's used as horizontal strike, the Ulnar Nerve would strike the target, not the elbow.