The only explanation for the bidding on this deal is that North got carried away. Think about your line of play before looking at the complete deal.
West led the ♦K, ruffed in dummy while East played the queen. How would you continue in this most optimistic contract?
At the table, declarer stopped to assess the situation. East appeared to have the ♦Q J, so that meant that West was likely to hold all the missing high-card points. Declarer desperately needed an entry to hand to take the club finesse. The problem was how to accomplish that.
If trumps were 2-2, the ♥7 would provide an entry, so declarer cashed two top hearts in dummy. West showed out on the second heart, so that hope was dashed.
Considering the deal further, declarer surmised that diamonds were 5-4, with West having five. South knew that West could hold four spades at most, but it was unlikely West held length in both minors or he probably would have bid again.
Finally, South pinned his hopes on West’s holding 4-1-5-3 shape. Backing his judgment, declarer played the ♠J from dummy – the perfect solution, as you can see from examining the full deal: