Usually in baseball, people talk about a player being the spark plug or the igniter. That means that this player starts the “rally”–a really big half-inning. The Baseball Dictionary says that a rally is a “run scoring surge during a half inning for the offensive team that causes it to tie or take the league.” Or, as Kansas City Royals announcer/witticist Rex Hudler would say, they drive the bus.
It takes a special kind of talent and personality to rally a team when you are facing almost certain elimination when down 6-2 in the top of the eighth.