It comes from the game of cricket.
Two batsmen are on the field at the one time. The one "at the crease" is waiting to receive the next delivery from the bowler.
So he is the one in action at that point in the play. As an analogy, it can be used to describe someone in action, on the spot, "on the job" (no jokes necessary here), someone of responsibility.
In other words, what Fresno said.