For me, personally, it’s the divisible-by-three check. You know, the little shortcut you can do where you add up the individual digits of a number and if the resulting sum is divisible by three, then so is the original number.

That, to me, is black magic fuckery. Much like everything else in this thread I have no idea how it works, but unlike everything else is actually a handy trick that I use semifrequently