Discarding Losers on Dummy's Winners
When dummy has more winners in a suit than you have cards, use the extras to throw your losers, and do it before the defence gets the lead.
When dummy holds more winners in a suit than you hold cards in it, the extra winners let you throw away losers from your hand. The timing matters: take the discards before you give the defenders the lead.
Why it works
Once your hand is out of a suit, each further winner dummy cashes lets you discard a card from another suit. A loser thrown away cannot be won by the defence. But if the defence gets in first, they will cash those tricks before you can throw them.
A worked example
You are in 4♥. West leads the ♦K.
Dummy (North): ♠ A K Q J ♥ J 9 5 ♦ 8 6 ♣ 7 6 3 2 (11 HCP)
Declarer (South): ♠ 7 2 ♥ A K Q 10 8 ♦ A 5 4 2 ♣ K 4 (16 HCP)
You have three diamond losers and up to two club losers. Your tricks: five hearts, four spades and the ♦A make ten, but only if the spades count. You hold two spades, so the ♠Q and ♠J each give you a discard.
Win the ♦A and draw trumps in three rounds, which needs only a normal 3-2 split. Then lead a spade to the ace, cash the king, and on the ♠Q and ♠J throw two diamonds. Now you lose at most one diamond and two clubs: ten tricks.
The losing line is to lead a club towards your king first. If East has the ♣A, the defence wins it and cashes two diamond tricks. With the ♣A as well, that's three tricks for them before your discards arrive. Add a second club loser and you are one down.
Four common mistakes
Starting on a suit that loses the lead while your losers are still in your hand.
Not counting how many cards you hold in dummy's long suit, so you miss the discards.
Discarding the wrong card: throw the suit where the defence can take tricks, not a suit where you might still win a trick.
Cashing side winners before trumps are drawn when a defender could ruff one.
When the rule does not apply
If drawing trumps would give up the lead (for example, you are missing the trump ace), you may need to take the discards first and accept a small risk of a ruff. If a defender is likely to be short in dummy's suit, draw trumps first. Count the risk both ways.
Key takeaways
Extra winners in dummy's long suit mean discards for you.
Take the discards before you give up the lead.
Draw trumps first when you can do so without losing the lead.
Throw the cards the defence is about to win.