It's a betting feature of Blackjack, a casino banking game.
Bet Behind in blackjack is a side bet that allows you to wager on the outcome of another player's hand. Simply click on another player's hand if they're participating and select a stake as you would for any other blackjack side bet. If the player wins in the following round, you'll double your stake. If not, the stake is lost.

The best part is that you can bet behind blackjack even if you don't have a seat at the table. In fact, that's what the feature was designed for. Of course, you can also bet behind other players if you have a seat. For instance, some players enjoy playing multiple hands at a time but may be unable to due to overcrowding.

"Bet behind is a feature that allows players to bet on other players' hands. If the table is full or a player wants to take a break, they can still participate in the game by betting on the hand of another player who is actively participating in the game. The bet behind feature is available in many online casinos and is becoming increasingly popular."
(Source: https://www.casinopapa.co.uk/casino/bet-behind/)

"Bet Behind is an exciting feature offered in Live Blackjack games that allow players to place bets on another player’s hand. This feature is especially useful when all seats are taken or when players want to sit back and enjoy the game without actively participating."
(Source: https://www.livecasinocomparer.com/live-casino-games/live-bl...

"Bet Behind is a feature offered in Live Blackjack games which enables a virtually unlimited number of players to participate in one game. The Bet Behind feature allows players to bet on the outcome of other players’ hands, while waiting for a seat at the table or simply wishing to join the action."
(Source: https://www.casino.com/uk/live/blackjack/bet-behind/)

So in the above-quoted context: The Urdu translation "فعال کھلاڑی کے ہاتھ پر شرط کا موڈ" can be understood as "a mode of betting on the hand of an active player." However, to accurately translate "the bet-behind mode" in the given context, a more appropriate Urdu translation could be:
"فعال کھلاڑی کی شرطوں پر شرط لگانے کا موڈ" which means "a mode of placing bets on the active player's bets.

So what should the asker do while translating this term into Urdu? He can adopt two ways
1. Use the suggested translation + English term in parenthesis this way;
فعال کھلاڑی کی شرطوں پر شرط لگانے کا موڈ (the bet-behind mode)
2. Use the suggested translation straight away;
فعال کھلاڑی کی شرطوں پر شرط لگانے کا موڈ

The suggested translation not only translated the term "the bet-behind mode" but also described it so the reader could understand easily what this term refers to: