When one chick accidentally slaps another chick's vagina while attempting to slap her butt from behind.
When two chicks are dancing and one goes to slap the other's butt from behind and accidentally swings through the legs reaching her vagina. This is called the "slap through."
first girl: OMG! I'm so sorry! I didn't mean to slap through.
second girl: It's okay. I kind of liked it. ;)