Here's the theory behind combustion. Let's say you have a 20% chance to crit. So, if you walk out and cast a spell (we'll say fireball), at a rabbit, 1 in 5 should crit. Okay, now let's say you have 10 rabbits lined up. You pop combustion and hit the first rabbit. You have a 20% chance to crit. You hit the rabbit and don't crit. You target rabbit #2. Now you have a 30% chance to crit. Let's say you crit (so you've used 1 of the 3 charges). You target the next rabbit and you have a 40% chance to crit. You do, so now you only have 1 charge left. Now you target the next rabbit and you have a 50% chance to crit. You don't. Next rabbit you have a 60% chance to crit. And so on until you have critted 3 times, then combustion is used up.
In regards to the question... the thing that crits is the spell you cast (e.g. if I cast a fireball, then instead of a normal hit, you get a critical hit for more damage and it catches on fire and squirms around a bit more).
HTH
Edited, Nov 20th 2007 9:43am by ktangent