Source:          Here we see a Loser on- Loser play used in a 4-3 fit (in order to preserve the – precarious – trump length). South Deals None Vul
  1. South is aware that partner may have just three-card support (perfectly admissible for a single raise, although not for a jump raise such as 1-3 or 1-4 ). However he is not keen to play notrumps with the opposing hearts unstopped. Perhaps his sturdy four-card spade fit will survive the 4-3 fit…

What happened West led A, and, much more sensible given his four-card trump length, did not switch to his singleton diamond, rather playing out two more top hearts. A trump short of the normal eight-card fit, declarer was not thinking too lucidly. He trumped the third heart, then played out three top trumps, hoping for an even split.

No good – East discarded on the third trump. This left West with the only trump in the pack – bad news. A heavy-hearted declarer turned to diamonds. West ruffed the second round, and cashed two long hearts. Down two.

What should have happened  Six missing trumps do not normally split 3-3, so declarer should have been very reluctant to ruff the third heart, shortening the long trump holding. [Admittedly he had then been unlucky that it was West, with the long hearts, who held the four trumps, but even if East had held them, the game would have failed, provided East delayed ruffing a diamond until the fourth round, thus severing declarer from dummy’s long card.]

Look at things another way: declarer has ten top tricks via four trumps, five diamonds, plus the ace of clubs. If he discards at Trick Three, the key Loser-on-Loser play, he retains those ten winners. He can win any return from West (ruffing a fourth heart high in dummy), draw the trumps, then run all his winners. 10 tricks and game made.

If you remember one thing…  Look out for Loser-on-Loser plays to preserve trump control – especially in 4-3 fits.