Bonus hunting is the intentional abuse of bonuses, in order to make a long term profit. The golden era of bonus hunting was 10 years ago, when it was enough to know how to play Blackjack, and players could get to a decent amount of winnings from bonuses alone.
Naturally, casinos try to prevent such abuse, and so today, bonus hunting is harder and more risky than before. Contemporary bonus hunting is mainly about the high variance of game strategy. In other words, it could lead you to either lose everything, or win at least 20-times the bonus amount. It requires a good starting capital, a clever choice of games and game strategy, as well as a thorough knowledge of the terms and conditions. The main risk is that on average, you only convert 1 bonus out of 25, and then if casino refuses to payout that bonus, you don't win anything. Some other casinos wouldn’t even give you back your deposit.