Using the standard 'bases' of sexual advancement...
rushing the mound refers to "forgetting the other actions/activities defined by the 'bases' and going straight for anal sex"
This action is quite looked down upon, in fact, the result is usually immediate ejection from the field of play.
The other day, Shawn got pissed off because his girlfriend, Leebs, was not putting out. He broke up with her, found another girl from the club and rushed the mound. He got what he wanted, he got a nut, but afterwards she told him to "get the fuck out of my apartment" and promptly slapped him.