To discard one losing card on another, as illustrated in the following examples:
As South, you play in 4♠ and receive the lead of the ♦K. If East obtains the lead with the ♥K, there is the danger of three club losers in addition to a heart loser.
Therefore, after drawing trumps ending in dummy, you play dummy’s ♦J (you can presume from the lead that West holds the ♦Q) and discard a losing heart.
You can then establish heart tricks by taking a ruffing finesse into the safe hand.
Playing in 6♥, you receive the lead of the ♦K. If you try to ruff the third or fourth round of clubs low, you will incur an overruff and subsequently lose a spade.
You can, however, ruff a club once with the jack and then return to hand with a spade to play another club; but this time, instead of ruffing, you make the loser on loser play of discarding your remaining spade from dummy. Dummy can then ruff a losing spade in safety.
South plays in 4♠ and West leads three rounds of top hearts. If you ruff the third round, you will lose control of the trump suit on the probable 4-2 break.
You should, therefore, discard a loser in clubs on the third round and dummy’s trump holding is able to take care of any continuation in the suit.
There are many other variations when the loser on loser play is good declarer technique.