“Dirty pool” has become a slang expression in the English language for doing something devious or underhanded. What “dirty pool” sometimes means in the world of pool is a player committing an intentional foul. Why would you want to do such a thing? Well, really for the same reason you would play a safety: to protect your next move when you have no other options on your current play. It's all about strategy.

You would resort to intentional fouls when all else fails. Obviously your immediate goal is to pocket the balls on the table. If you can't pocket a ball, you could choose to play a safety. But sometimes you may not be able to do either, and the only way out would be to commit an intentional foul. You'll need to be playing pool for a while before you can start to understand when and why you should intentionally foul. If you're in a tournament or a league, it's a generally accepted strategic move, but if you're playing one-on-one, say, in your local bar, you'd better make sure you know your opponent well enough, or you could find yourself in a barroom brawl!

If you're playing by official rules, you'll have to keep one foot on the floor when you shoot your shots. That does mean though, that it doesn't have to be your entire foot. It could just be your big toe if you're really stretching across the table. If you lift both feet off the floor when shooting, it's a foul and your opponent gets either ball-in-hand or ball-in-hand behind the head string (see definition above), depending on the game — another reason for you to be clear on the rules before you commence your match.