One of our self defense key chains is called the Monkey Fist. It's a 10 inch braided paracord keychain with a wrapped steel ball on one end.
Sometimes these are called biker's whips or slungshots and they can be very useful if you're in a situation where you're confronted by someone trying to mug you or attack you. The steel ball adds inertia, allowing the 10 inch rope to unleash a nasty crack.
Here are a few tips you can use once you receive your Monkey Fist.
Understand Distance - When an attacker comes at you, they're going to try to get in close. With this key chain you're going to have some added leverage. Understand how much distance you need to make a good strike.
Practice Your Swings - You can swing the weapon down, up, or from side to side. The most important thing to focus on is accuracy and where the ball will go if you miss. Use swings that arc in a way that doesn't put your body at the end of a strike.
Movement - Continue to weave and strike. Just because you have a weapon, doesn't mean you should stand still and start flailing. Watch this video on head movement, and try to incorporate it into your practice sessions.