If you bet a 4-leg parlay and the first three have hit, you can hedge by betting against the 4th leg — guaranteeing some profit regardless of outcome. Common in futures markets (championship bets) as the field narrows.
Advanced concepts
Hedge
Placing a second bet on the opposite side to lock in profit or reduce exposure.
Example
You bet +2000 Celtics to win the NBA title at the start of the playoffs. At the Finals, you bet the opponent's moneyline to lock in profit either way.