For a moment, turn things around and think about trumping a loser in your hand — the long hand. Let’s put this theory into practice. Assume that hearts is your trump suit.
You want to draw trump, so you play ♥AKQ, removing all your opponents’ trump cards. You remain with ♥J2, both of which are winners. You score five heart tricks. Agreed?
Now see what happens if your opponents lead a suit that you don’t have and you trump the lead with your ♥2. You remain with the ♥AKQJ, four tricks, plus the deuce you have already used. Same five trump tricks. Trumping with the ♥2 in the long hand doesn’t give you an extra trick. But if you can manage to trump a loser in the dummy, the short hand, you still have five winning heart tricks in your hand plus the ruff (trump) in the dummy. Six trump tricks!