…I suggest you don’t use it.
Dungeons and Dragons, and RPGs of similar cast, are relatively unique in the annals of games, in that they pit one player (the GM) against all the other players (the PCs) and the GM has the ability to quite literally throw everything and anything he can at them to thwart their progress towards their goals.
Sweet Reason, if you could play chess like that, or Ogre, or Panzerblitz, it would never work. Nobody would ever want to play the side that didn’t have the “I can do anything I want, ’cause I’m the GM” power.
So what is it about RPGs that lets the GM get away with it? Quite simply, RPGs have a different goal than purely competitive games such as chess or Ogre or Squad Leader. There is a shared understanding between the GM and the PCs that the challenges the GM presents won’t be completely overwhelming. There will always be something that the PCs can do to survive (if not always win) the encounter. It may be obnoxiously difficult to figure out, but it’s always there.
SCENARIO A: “You enter the 30’x40′ room. As you reach the center of the room, the ceiling slams down at a speed of 500 mph, squashing you all into jelly. Please roll saving throws for all your magic items to see if they survived a ‘crushing blow’, for the next group of adventurers to find.”
SCENARIO B: “You enter the 30’x40′ room. As you reach the center of the room, the door behinds you slams shut and the ceiling begins to slowly descend. You estimate that you have 2 minutes before you’re all squashed into jelly. What are you going to do?”
You can tell instinctively that scenario B is going to be a lot more fun. Even if the GM hasn’t given the PCs any real clues about how to get out of this particular trap, the mere fact that they have the chance somehow makes it more fair. Maybe they’ll figure out that the triangular dagger they picked up in the previous room will, when fitted into the slot in the descending ceiling, stop its progress. Maybe they’ll figure out some use or combination of spells or magic items that the GM hasn’t thought of at all, allowing them to escape. Maybe they’ll get squashed into jelly. But at least they have a chance.
Adventures don’t have to be “fair” or “balanced”. Indeed, one of the fault lines between folks who like the newer generation of RPGs, as opposed to those who prefer older games, is that the latter don’t see anything wrong with an encounter that’s way over the ability of the PCs to overcome. Sometimes the winning play is to run away. The former sort would see that as somehow “losing” the encounter, and therefore see the encounter as inherently imbalanced and unfair.
One of the best qualities of a GM is that he is fair. Not in the sense that the PCs have a good chance (or even any chance) of “winning” every encounter, but in the sense that the PCs have a chance of surviving those encounters. Even though the key to surviving may be obscure, or the “survival” strategy is to run away from that ancient red dragon, it can still be done. A certain death trap, in this context, is unfair. An escapable death trap is fair.
Ultimately, I think it’s a lot more fun when the GM isn’t overly worried about making sure his encounters all have a chance for me to win. It’s all the more satisfying when I barely survive and live to tell the tale, or, better yet, come up with an outside-the-box idea that works. Knowing I have an 80% chance of “winning” every encounter is just… too balanced.