When should a manager challenge a call? That's a crucial element of baseball's new replay rule.MLB umpires Pat Hoberg (20) and Chris Guccione review a play challenged by San Francisco Giants manager Bruce Bochy (15) in the fourth inning during a baseball game against the Arizona Diamondbacks, Tuesday, April 1, 2014, in Phoenix.San Francisco Giants starting pitcher Matt Cain (18) throws in the first inning during a baseball game against the Arizona Diamondbacks, Tuesday, April 1, 2014, in Phoenix.Arizona Diamondbacks center fielder A.J. Pollock (11) beats the tag from San Francisco Giants starting pitcher Matt Cain (18) in the fourth inning during a baseball game, Tuesday, April 1, 2014, in Phoenix. Pollock scored on a passed ball.San Francisco Giants starting pitcher Matt Cain (18) talks to home plate umpire Eric Cooper (56) after Cain thought he tagged out a runner at home in the fourth inning during a baseball game, Tuesday, April 1, 2014, in Phoenix.