When you’re playing in a suit contract and you’ve seen your dummy, count your probable losers before playing a card. If you see too many losers, find a way to eliminate one of them.

Here’s a test case: Sitting South, with North dealing and North-South vulnerable, you hold:

♠️ A 10 9 5 2
♥️ Q 10
♦️ A J 8
♣️ K 7 6

The bidding proceeds as follows:

North          East         South         West
1♣️            Pass         1♠️            Pass
2NT            Pass         3♦️•           Pass
3♠️••         Pass         6♠️           All Pass
    
• New minor forcing, which asks if partner has three spades or four hearts

•• Shows three spades

Opening lead: ♦️3

Now you can see the dummy:

North
♠️ K J 3
♥️ A K 2
♦️ Q 7 4
♣️ A J 3 2    

You got a break and won with the ♦️J, but there still may be a loser
in the suit. In addition, you could lose finesses in spades and clubs. Any way to get rid of a loser?
Yes. Cash the ♥️Q, ♥️A, and ♥️K, then discard a diamond from your hand. Suddenly you have no diamond losers. As a result, you only need to win a finesse in spades or clubs to make the slam. That’s a 75% success rate.

Here are the hands:

                            NORTH
                           ♠️ K J 3
                           ♥️ A K 2
                           ♦️ Q 7 4
                           ♣️ A J 4 2

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

                          SOUTH
                         ♠️ A 10 9 5 2
                         ♥️ Q 10
                         ♦️ A J 8
                         ♣️ K 8 5

As you can see, the spade finesse works, and you’ve got your 12 tricks. What made it possible? The discard of the diamond loser.