In a normal fight, you're simply trying to hit your opponent anywhere and any way you can. However, sometimes you want more finesse - you want to hit your enemy in a particular spot (groin and head are two favorites). To aim at a particular area, simply indicate it in your `attack' command:
attack Quinn's head
This will initiate combat, with you aiming at that location. Keep in mind that it's a lot harder to hit your enemy when you're aiming so carefully. If, during a combat, you want to change targets, simply re-type the 'attack' command with a different target location (or none at all), or use the `aim' command:
aim for groin
You can also use the `aim' command outside of combat if you feel a special need to aim for a body part at all times.
To stop aiming, you can use any of:
unaim stop aiming aim for nothing