For example, the contract is 4♠, dummy holds:
There are only 8 top tricks available. To make the contract, 2 more tricks must come from ruffing.
The so-called "normal" line of play is to try to ruff 1 ♣ and 1 ♥ in dummy, but once the defenders realise the situation after the declarer losing the necessary tricks (a ♥ or a ♣), they will start drawing trumps themselves, defeating the contract.
However, by performing a dummy reversal, once the declarer gets to his hand, he goes to the dummy to win the ♦ ace, ruffing a ♦ in the hand, and goes to the dummy again using a ♥ and ruff a ♦ again, and goes to the dummy using a ♣ and ruff a ♦ again, then returns to the dummy using a trump, and ruff the last ♦. The trumps are then drawn, getting two more tricks and the contract is completed, no matter how the trump breaks.