Ruffing in the hand with the most number of trumps is usually fruitless. Those trumps will make tricks anyway.
North is in 4♥ with the ♠K lead.
You are a trick short. You have five hearts, one spade, and three clubs once you have taken the losing finesse in clubs. Where can you find that extra trick?
If you draw trumps you will make five tricks in hearts. If you ruff a diamond in the North hand, you will still make five tricks in hearts!
But, if you ruff spades with dummy's trumps, you will make six or seven trump tricks depending on the number of spades you ruff. If you ruff two spades, you will make an overtrick instead of going one down. Obviously, you should not draw trumps until you have used dummy's trumps to ruff spades.
Generally, it is only possible to score extra tricks in the trump suit by ruffing in the hand with the least number of trumps - which is usually dummy. The key is to look for a shortage in dummy. If there is a singleton or void, you might be able to use dummy's trumps for ruffing.
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