It is about a legal way to win, but take care! Such practice is considered an advantage gambling and, if the casinos see that you are a skillfull or knowledgeable player that can gain an advantage at blackjack by using the card-counting or shuffle tracking, at video poker by using a strategy card devised or not by computer analysis of the game, or at progressive slot machines by taking advantage of a high jackpot, you can end by being listed on the Black Book. And, if you will be listed finally on the Griffin Book shared by the casinos, you will end you career as gambler much sooner that you intended to do, because you will be excluded from all the casinos that have access to the book.
Bonus hunting (bonus bagging, or bonus whoring) makes possible from a mathematically point of view to get a profit. Wikipedia shows as example the house edge in blackjack which is roughly 0.5%. In the example above, $5000 in wagering with a house edge of 0.5% will result in an expected loss of $25. Since the player received a $100 signup bonus, after subtracting the expected loss of $25, the player has an expected profit of $75.
If you play at an online casino, you will take advantage by the signup bonuses, usually one-off bonuses for signing up to the casino and opening an account. There are terms and conditions for each bonus, like restricted games or bets and wagering requirements.
Wagering requirements prevent players from withdrawing the bonus money immediately after receiving it. Before the bonus money can be withdrawn the player must wager a certain amount of money on unrestricted games. The wagering can be spread out over many bets. To meet a $2000 wagering requirement, a player could make 1000 $2 bets on blackjack, provided blackjack is not a restricted game.
Games like blackjack and video poker have a low house advantage. The house advantage for blackjack is ~0.5%. So in the example above, by playing $2000 worth of blackjack with a house advantage of 0.5%, a player is, expected to lose $10 in total. But the expected value is an average value and may be less than the actual value.
Assuming that you only lose an amount close to the expected loss, and the bonus awarded to you after meeting this wagering requirement is greater than $10 then you will have a profit. The process is mathematically calculated. Bad luck could cause a player to lose more than $10 playing blackjack in this situation.
In sports betting, by taking advantage of free bet promotions, you can back the event, and then lay the event on a betting exchange, at similar or same odds, thus ensuring that the free bet is not a bet at all, instead more like a financial trade. No matter the outcome, it is possible (by using a betting / arb calculator) to calculate both the possible outcomes before the sporting event has started. By inputting the odds, the bet calculator then tells you what amount to bet, to ensure that whether the bet wins or loses, the return is the same.
In online poker, the poker player can take advantage by the incentive bonuses offered by the site after a certain number of raked hands are played. For example, a site may offer a player who deposits $100 a bonus of $50 once he plays 500 raked hands. A poker player who can at least break even can become a long-term winner by playing with poker bonuses. This way, a winning poker player can add to their winnings with the use of bonuses.