Taking Your Tricks Before Declarer's Discards Arrive
If dummy reveals a long, established suit, declarer will usually use it to discard their losers. You must take your defensive tricks actively before those discards happen.
When you are defending a suit contract, the sight of a long, strong side-suit in dummy should set off alarm bells. That long suit is declarer's primary tool for discarding losers. To defeat the contract, you must often race to take your defensive tricks before those discards can happen.
Why it works
Declarer usually needs to draw trumps and establish dummy's long suit (often by knocking out a defensive ace) before they can safely discard. This gives the defence a brief window of opportunity. By actively switching to the suits where you hold top honours, you can secure your tricks while declarer still holds losers in their hand.
A worked example: The race against dummy
The Contract: 4♠ by South.
Your Hand (West):
♠ 5 4
♥ A K 8 3
♦ 8 7 6
♣ J 9 5 2
8 HCP.
Dummy (North):
♠ Q J 10
♥ 6 5
♦ K Q J 10 9
♣ A 4 3
13 HCP.
You lead the ♥A. Partner plays the ♥2, and declarer plays the ♥7. You have won the first trick, but look at dummy's diamonds.
Declarer will surely use that K-Q-J-10-9 suit to discard their club losers once the ♦A is knocked out. If partner holds the ♦A, declarer's plan will work perfectly unless you intervene. If you cash the ♥K and then play passively, declarer will draw trumps, knock out the ♦A, and throw away their clubs. You must switch to clubs now at trick two, hoping partner holds the ♣K or ♣Q, establishing your club tricks before the diamond discards arrive.
Four common mistakes
Continuing the suit you led just because it feels safe, ignoring dummy's threat.
Fearing giving away an overtrick when the contract is making if you don't attack.
Switching to a suit where dummy has a singleton, giving declarer a free ruff.
Delaying the attack until declarer has already drawn trumps.
When the rule does not apply
If you hold the stopper in dummy's long suit (for example, you hold the ♦A in the example above), there is no rush. Declarer cannot run the suit without letting you in first. In this case, you can often afford to defend passively and avoid making risky attacking leads.
Key takeaways
A long, strong side-suit in dummy is a major threat.
Declarer will use it to discard their losers.
Attack suits where you need to build tricks immediately.
If you control dummy's long suit with the master card, you can afford to wait.