Online casino promotions like sign-up bonuses work by offering players extra funds or free bets when they sign up and deposit money. For example, a casino might offer a $100 sign-up bonus with a $100 deposit. These bonuses are designed to incentivize players to join and continue playing on the platform. Knowledgeable players can use these bonuses to place bets that are guaranteed to win, such as betting on all colors in roulette, thus turning the bonus into real money without spending their own funds.