Rivalry increases base power of a move by 25% if the foe and the user have the same gender, and reduces base power by 25% if the foe and the user have opposite genders.
If the user or the foe is genderless, the move's base power is unaffected.
It works nicely on Pokemon Online, since nobody bothers to check gender, but most of the pokemon that get it have much better abilities for their jobs.
