During the play of a contract, it can be dangerous to lose the lead to one opponent but safe to lose it to the other. The hand that declarer needs to keep off lead is the ‘danger hand’. Sometimes you can organise life accordingly:
West leads the ♠J against 3NT. The contract is at risk if the defenders make four spade tricks and one of the minor-suit kings. You have a choice of finesses to take. At this stage, it is unclear which defender will be the danger hand, the hand to whom you do not want to lose the lead.
Dummy covers with the ♠Q, which holds the trick. Since the ♠K is vulnerable to a lead through it, East becomes the danger hand. Accordingly, you take the diamond finesse into the West hand.
If East held the ♠A, the ♠Q would not hold (swap the ♠A and ♠7). You would duck the second spade and win the third round with the ♠K. West, with the long spades, would be the danger hand and you would instead finesse clubs into East.
