This question originally appeared on Quora. 
Answer by David Chan, MD and Black Belt Shotokan Karate
There's a fair amount of power behind the kick because of hip and body rotation. And because it's coming from the side, it's one of the more difficult kicks for the opponent to catch and throw.
The downside risk is injury to the ankle or knee of the attacker if performed improperly. There can be injury to the kicker's striking ankle. If the attacker uses the ball of the foot as the striking surface, it's fairly safe, but also harder to be accurate with the strike. If the top of the foot is used to the head, the ankle can become hyper-extended and sprained. Often attackers try to use the lower part of the shin to hit the opponents head or front leg, and that's effective. But if the opponent moves backward, the top end of the foot could become the striking surface, unintentionally putting undue stress on the ankle.
There is also potential of injury of the kicker's standing knee depending on ground surface or footwear. Knee injury can occur during the rotation if the standing foot sticks to the ground. Practitioners often kick off the ball or heel of the standing leg rather than let the foot remain planted to prevent this kind of rotational injury. This is really important if wearing shoes. Rotational injury of the knee could result in ligament sprain or even tear.