The great Mark Grace (before you ask, former Cub/Diamondback) came up with the term when he played in Chicago years ago... He defined it as...
"A slumpbuster is if a team's in a slump, or if you personally are in a slump, you gotta find the fattest, gnarliest, grossest chick and you just gotta lay the wood to her. And when you do that, you're just gonna have instant success. And it could also be called jumping on a grenade for the team."