South must not be tempted to ruff here !!
If he does Declarer will discard the 3♣. If South then leads a red suit next, Declarer will ruff and Dummy will discard his 4♣ (colloquially known as a "ruff and sluff").
If South leads a ♣ instead (away from his Q♣) Declarer will play his 5♣ and win the trick with Dummy's J♣.
In either case NS will win no other trick in the game.
By discarding his Q♦ South will ensure he will win 2 more tricks : the Q♣ and his trump winner (Q♠) !
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