The idea is basically that (something + 12) + (something else - 12) is the same as "something + something else". You're taking 12 away from the threes, and adding it to the fours. Or vice versa.
If this doesn't make sense, there's no reason to waste time pursuing it any further, because, in any situation where it might apply, it's easy enough just to test numbers instead.
(In fact, I'd argue that it's better not to remember things like this -- even if your mind is capable of that kind of feat -- because, if your mind is too cluttered with this kind of stuff, you'll lose mental flexibility.)