Lots of bets on horse A. Makes the odds on horse B look cheap. Place late bets on B.

It helps if you can also rig the race, but just knowing the odds better than the pari-mutuel stakes gives you an advantage.

But if it's parimutuel, don't you lost more on A than you can make on B? Are you tricking other players, or is the idea that you make lots of bets on A, and then make bets on B fast enough that the bookmaker does not have time to correct?