This is the same as a dummy reversal. It involves taking multiple ruffs in the long trump hand, thereby generating an extra trump winner, while leaving the short trump hand to draw the opposing trumps.
Suppose you have a trump suit of A-Q-10-x-x facing K-J-9. If you take three (or more ruffs) in the long trump hand, you will make more than the five trump tricks with which you started. For reversing the dummy to be successful, you will usually need strong trumps and plenty of entries in the short trump hand.
Another good reason for reversing the dummy is that dummy’s trumps are strong but yours are not. For example, with a trump suit of 6-5-4-3-2 facing A-K-Q, ruffing with the high short trumps is going to be unproductive.