It means don't bet against huge favorites when their is negative pool. This happens alot when somebody bets $100,000 to show on a sure thing, and there's only $105,000 in the total show pool. If the sure thing runs out the show payoffs on the top 3 finishers are extremely large. By the bettor making hedge bets with the bookie in these situatuions your bet will not be included in the total show pool and the the prices on the top 3 finishers will not be diminished by your show bets.
