Cashing Out to Beat the Contract
When dummy has a long running suit, declarer will discard losers as soon as they get in. Count, then take your winners first.
Patient defence is usually right. But when declarer has a source of tricks that will run as soon as they get the lead, waiting can cost the contract. Then you must take your winners while you can.
Spot the danger
Look at dummy. A long, solid suit such as A K Q J 10 means declarer can throw losers away as soon as trumps are drawn. Any trick you have not taken by then may disappear.
A worked example
South plays 4♥. You are West and lead the ♠A from A K 7.
Dummy (North): ♠ 8 6 ♥ K 7 3 ♦ 9 4 2 ♣ A K Q J 10 (13 HCP)
Count. Dummy's clubs will give declarer five tricks and plenty of discards. The defence needs four tricks, and they must come quickly.
You have two spade tricks: cash the ♠K too. Now you need two more. You also hold ♦ K J 5, so diamonds are the hope. Switch to a low diamond at trick three. If partner holds the ♦A, they win it and return a diamond through declarer's queen, and your K J take another trick. That is four tricks before declarer can discard a diamond on the clubs. If you wait, for example by leading a trump, declarer draws trumps and throws the diamond losers on dummy's clubs.
Four common mistakes
Making a "safe" passive lead when dummy has a suit that will run.
Not counting how many tricks the defence needs.
Cashing one winner, then switching away and never getting back in.
Forgetting that partner may hold the winners you need: lead through declarer's strength.
Key takeaways
A long, solid suit in dummy is a warning: take your tricks quickly.
Count how many tricks you need and where they can come from.
Cash out or attack before declarer gets the lead and starts discarding.