Knowing when to surrender in Blackjack involves understanding the dealer's upcard and your own hand. Generally, players should consider surrendering if they have a weak hand, typically a total of 15 or 16, while facing a strong dealer upcard, particularly when the dealer shows a 10 or an Ace. The rationale behind this strategy is that the chances of improving your hand are low, while the dealer is likely to have a strong hand that can beat yours if you continue playing. Surrendering allows you to minimize losses by forfeiting half your bet rather than risking it all in a challenging situation.