Dice Mechanic

Nearly all tabletop roleplaying systems use dice in some form or another to show chance. World of Darkness uses d10's (ten-sided dice) exclusively, Risus uses d6's (six-sided dice) exclusively, and the d20 system uses all sorts of dice, including d6's, d4's, and the ever present d20. In fact, the basis of nearly every d20 system is the d20 itself, the twenty-sided die, which is said to be the best indicator of luck and chance.

Well, it's not.

Numerous people have said this, not the least of which was Wizard's of the Coast themselves in their book Unearthed Arcana. In it, they offered up a variant mechanic, which replaces all 1d20 rolls with a 3d6 roll. The reasoning is that, while 1d20 offers up a possible roll of 1 to 20, it gives that chance to everyone. It feels much less like reality, because a barbarian with 18 Strength can lose an arm wrestling match to a milk maid with 8 Strength just because of a lucky roll (for reference, the milk maid would just need to roll five higher than the barbarian to beat him; not that hard).

A roll of 3d6 changes the dynamic, where each of the different possible results (3 to 18) have a bell curve distribution of probability, instead of the straight, linear probability of 1d20 (where you have a 5% of rolling any of the numbers). The exact distribution can be seen below. Now, what does this do exactly? Well, first off, it makes it much more likely that people will roll closer to the average, which is 10 or 11. That in an of itself is very good for a game such as this, because it makes the game more dependent on actual skill (which, as you might have noted, is a significant goal of ReD&D). Secondly, it also changes several different mechanics, such as critical hits and critical failures.

Explosions & Implosions
To make the 3d6 system work even better, the explosion/implosion rules were added. What this means is that, if someone rolls three 1's or three 6's on a single check, the roll either "explodes" or "implodes," depending on whether it was three 1's or three 6's.

Explosion
Rolling three 6's at once explodes a roll. Normally the result would be an 18, but instead, you roll the dice again, and then add the second roll to the first. So, essentially, on an exploding roll, you are rolling 6d6, and adding them all together. And, if you happen to, by some stroke of luck, roll all 6's again, the explosion explodes, and you get to roll again. This can technically go on indefinitely, but if it ever happens more than three times in a row, I would check for rigged dice.

Implosion
On a roll of three 1's, the roll implodes. An implosion is not an automatic failure, but it is much more likely to fail. When a roll implodes, you roll 3d6 again and minus it from the check. Now, what this means is that an imploding roll ends up granting a penalty from -1 to -14 on the check. Obviously not a very pretty result.

Compounded 'Plosions
It is possible, though highly unlikely, to roll an explosion or implosion after already having rolled an explosion or implosion. This unusual circumstance works pretty much in the same way, but an explanation would likely make things easier to understand. We'll start with an explosion-explosion, because it's the easiest.

Explosion-Explosion
And explosion-explosion is check where you roll three 6's and then, when you roll again, you roll three 6's again. This is pretty much just an awesome bit of luck, because you get to roll 3d6 again. So, essentially, an explosion-explosion is likely rolling 9d6.

Explosion-Implosion
An explosion-implosion is a check where you roll three 6's and then, when you roll again, you roll three 1's. This is an unfortunate result, but still pretty easy to understand. When you roll a third time, that roll is subtracted from the total result. The order of operations would look something like this: 7 + 18 + 3 + (-11) = 17.

Implosion-Explosion
An implosion-explosion is a check where you roll three 1's and then, when you roll again, you roll three 6's. This is a very unfortunate check, because it results in a terribly low roll. When you roll the third time, that roll is added to the negative result, making it even lower. The order or operations would look something like this: 7 + 3 + (-18 + -11) = -19. A very low roll indeed.

Implosion-Implosion
An implosion-implosion is a check where you roll three 1's and then, when you roll again, you roll three 1's again. This is actually an exceedingly lucky roll, and is probably the best thing that can happen if you roll an implosion. When you roll the third time, that roll is subtracted from your negative roll, resulting in a positive result. The order of operations would look something like this: 7 + 3 + (-3 + 11) = 18.

Further 'Plosions
Technically, it's possible to compound explosions and implosions even further than this, but the probabilities of such results are exceedingly, exceedingly low. As it stands, rolling either an explosion or implosion happens 1 out of 216 times each, so every 108 rolls you make will, on average, result in either an explosion or an implosion. However, to get a single compound check only happens 1 out of every 11,664 times. That's pretty unlikely right there.

Let's go further. Your chances of getting the following rolls are as follows: So, there you have it. The probabilities of getting these highly compounded 'plosions is very, very small. This is included only because it is referenced in the skill tiers, particularly in the skill tiers above Epic.