When a free bet wins, your sportsbook will only give you the profit from the wager and not the stake. This is known in the industry as a Stake Not Returned (SNR) free bet. Whereas your successful real money bets will see your profit and stake returned to your account.
The rules are the same as standard Blackjack, but Guests may split and double down a single time each hand for free. If your hand wins, the original wager as well as the Free Bet will be paid. If your hand loses, you only lose the original bet.