Horse racing is a game of uncertainties -- most of the time. But there are some rare situations in which an ordinary bettor makes a guaranteed profit, with no risk.

A New York gambler named Howie is a student of these special situations and he found one at Aqueduct last Saturday. It had all the necessary ingredients; a small field and a standout favorite who was bet very heavily to show.

Across the country there are bettors, known popularly as bridge-jumpers, who will wager tens of thousands of dollars to show on supposedly unbeatable horses. Sometimes the public at large will bet tremendous sums to show on a standout in order to collect the minimum payoff of $2.10 for $2. But whenever a horse takes almost all of the show money in a five- or six-horse race, a gambler may be able to create for himself a virtual sure thing.

Harry started formulating his show-betting strategy in 1979, when New Yorkers would invest hundreds of thousands of dollars to show on the great filly Davona Dale whenever she raced. He could usually anticipate the races in which opportunity would knock and, he said, "On Saturday, I knew the situation was going to arise. I got my bankroll and I was all set." kClever Trick was the overwhelming favorite in the Paumonok Handicap, and was facing only four rivals. He looked like a certainty to finish in the money. Accordingly, an avalanche of money poured onto him in the show pool.

If such a standout favorite finishes in the money, a gambler who bets $10,000 on him to show will collect a $500 profit. But to guard against his finishing out of the money, the gambler can bet this $500 on the other horses in the race to insure himself of a profit, anyway. "You can guarantee yourself about a 3 percent return," Howie said.

A total of $201,795 was wagered to show on the Paumonok Handicap, with this breakdown: Clever Trick . . . $195,133 Judge Grey . . . $1,077 Petite Roche . . . $1,530 Dunham's Gift . . . $2,616 Dr. Blum . . . $1,439

A few minutes before post time, Howie calculated the possible show payoffs in the event that Clever Trick ran out of the money. (For the uninitiated, this is how to figure a horse's show payoff: subtract 17 percent -- the track's cut -- from the total show pool. Then subtract the amount of money wagered on the in-the-money finishers. Divide this amount by the sum bet on the horse whose payoff you are calculating. Multiply by two. And add $2. It's not as complicated as it sounds.)

After performing this arithmetic a few minutes before post time, Howie made $10,520 in show wagers, as follows: Clever Trick . . . $10,000 Judge Grey . . . $60 Petite Roche . . . $80 Dunham's Gift . . . $140 Dr. Blum . . . $240

Howie had applied some handicapping judgments to this wager, figuring that Dr. Blum looked like the most reliable of the contenders.With this combination of bets, the worst thing that could have happened would have been a one-two-three finish of Clever Trick, Petite Roche and Judge Grey. In that event Howie would collect $10,500 on his Clever Trick bet, $84 on Petite Roche and $61.20 on Judge Grey, a net return of $10,645.20.This would represent a return of $145.20.

What happened, however, was far more eye-catching. Clever Trick ran an inexplicably dull race and finished fourth. Because there was so much money in the show pool to be distributed among so few ticketholders, the Aqueduct tote board flashed some remarkable payoffs: Dr. Blum . . . $13.20, $5.60, $77 Dunham's Gift . . . $4.80, $3.20 Petite Roche . . . $72.40

Howie lost his big show bet but still collected $15,160, a risk-free profit of $4,640. He recognized, in retrospect, that he might have adopted an even more lucrative strategy, settling to break even if the favorite finished in the money and shooting for a major killing if he ran out. Still, he was satisfied that he had executed one of the ingenious types of racetrack betting strategies. And he could leave the track saying, with justification, "They were giving money away today!"