The reason North felt safe in supporting his partner’s ♣s with only three cards was that South had passed over East’s 2♥ bid implying a minimum opener, and was thus almost certain to hold five ♣s.
West led ♥6 and the defence played three rounds of ♥s, declarer trumping the third. If he had drawn all the opposing trumps at this point he would have gone down - with only one trump remaining and two aces to dislodge. Instead he led ♠Q at trick 4. West won ♠A but, with dummy trumping ♥s, played a second ♠. Winning dummy’s ♠K, declarer drew all the trumps, then forced out ♦A. He made his contract - as would 2♥ by the opposition.

ANDREW’S TIP: Play a minimum opener to have five + cards in the suit opened. (Assuming a Weak Notrump is being played.)