ICM... how does it work?

[proell]

Is there an ICM for dummies somewhere?
I've read the "How to use ICM" for dummies, but was wondering how the calculator actually works. I have been looking at it the past few days, and now have gotten curious as to how it gets its percentages exactly.

I tried to find the formula in other posts but have had no luck. It's probably all over my head anyway, but I'd like to take a shot at understanding the mechanics behind it rather than just using it blindly.