Explain xkcd

Date: July 25th, 2011
From: Dan Hoey
Subject: Speculation

Harm wrote:

  In this respect, can the ball be metaphorical in the sense of
  "passing the ball" in a conversation?

Or possibly "catching" as in "sexually submissive" is metaphorical for
"didactically submissive."  Black Hat writes his own rules:  he pitches.