(Note that this comic came out less than a week after Super Bowl XLIII, the final game of the 2008 NFL season, which was played on 2009-02-01).
Here is a translation of Ponytails comment: This would mean that Ponytail was about to go down on another girl, while this girl had her period.
Then Ponytail begins with yet another base analogy by mentioning crossing the pitcher's mound, but then suddenly she brings two more, very different, sports into the metaphor: American football (with the 50 yard line) and bowling (with the ten-pin). Ponytail then brings up a third sport in her elaboration; her reference to getting a "red flag" could refer to different sports, as many sports use flags, some of them red.
It is, however, most likely a reference to racing in motor sport as the red flag is displayed when conditions are too dangerous to continue the session.
They fumble around with the definition in panel two where Ponytail ask is that Below the waist, but... Cueball tries to put this into the base system and suggest that this could be compared to the difficult shortstop fielding position in baseball, between 2nd and 3rd base.