Bridge players constantly re-evaluate their hands as the bidding progresses. Here’s a example, with South dealing and North-South vulnerable:

                   ♠️ A J 9 4
                   ♥️ A Q 8 3
                  ♦️ A 8 3
                  ♣️ K 6

WEST                             EAST
♠️ 6                              ♠️ Q 8 5 2
♥️  K J 7                      ♥️  6 5
♦️ J 10 7                      ♦️ 9 5 4 2
♣️ Q 10 9 7 3 2            ♣️ J 8 4

                  ♠️ K 10 7 3
                  ♥️  10 9 4 2
                  ♦️ K Q 6
                  ♣️ A 5

The bidding proceeded as follows:

West           North         East         South
Pass            1♥️           Pass         2♥️
Pass            3NT           Pass         4♥️
Pass            4♠️•          Pass         4NT••
Pass            6♥️          All Pass
• Roman Key Card Blackwood
•• One or four key cards

Opening lead: ♣️4

Experts have devised a new slam-bidding system that begins at a low enough level that the partnership doesn’t go too high. The bidding starts at one suit higher than the trump suit, usually at the four level. Hence,4♠️. Replies go one level higher at a time. Here 4NT means one or four key cards, 5♣️ zero or three, 5♦️ two without the trump queen and 5♥️ two with the trump queen. The five key cards are four aces and the trump king.

What was North thinking in the bidding?  The 3NT bid asked if South raised with four hearts. If not, South would pass. It was unlikely that South held enough high-card points for North to consider a NoTrump slam at this point. Once an eight-card fit is established by the 4♥️ bid, North’s already strong hand now has a trump suit. It’s time to investigate slam.

North had to play carefully to bring home the slam. After winning Trick One with the ♣️A, North led the ♥️10 for a possible repeat finesse. West covered with the ♥️J (better to wait until the ♥️9 is led ): 10, J Q 5. When the ♥️K didn’t appear, declarer’s path was clear. Lead a diamond to the board and finesse with the ♥️9, followed by the ♥️A dropping the ♥️K. South eventually lost a spade finesse but made the contract.