Advantage d100
The top hit when you search for this is some bottom bin AI slop right now. So I figured someone outta post this.
Warhammer Fantasy Roleplaying is the main game that uses advantage d100 (also called flip to succeed). Related systems like Mythras and Zweihander also make use of the concept. Could go all the way back to BRP. I'm not gonna check, are you?
Advantage d100 is when you roll d100 as normal, but flip the digits if it would be better for you.
So in the usual roll at or under procedure 10 & 0 (read as 10) becomes 1. 30 & 2 becomes 23. 40 & 9 remains 49, better than 94. 20 & 2 remains 22, doubles stay the same as well.
The average roll is 35.66. Of course, a standard d100 would be 50.5, so this a 14.84% improvement on average. But! The improvement is not constant. In fact, it's this weird spikey graph.
That bottom red line represents how much better you do compared to a straight d100 roll.
Play around with the anydice formula for yourself.
One last tidbit of interest is that this cuts down the number of possible results on the d100 to 55. So if you use Flip to Succeed to determine hit locations like WHFP, that's half as many entries you need to fill. Neat!