"Five Element Arm" is a series of self-defense techniques from the Practical Tai Chi Chuan system. It takes a specific scenario into account: what if you can't use both arms to defend yourself?
You might be injured during the fight.
You could be carrying something and not have the time to drop it.
You might need the other hand to keep track of the person you're protecting.
Whatever the reason you might find yourself in that scenario, this drill teaches you effective self-defense techniques for such an occasion. This video includes both the solo drill as well as detailed instruction on how to apply each technique.