East ruffs with the ten, and West's jack is promoted. The attack of the club suit killed a vital entry. Without that lead, declarer could afford to pitch the spade loser after the trumps are drawn. Indeed, it can. It may appear that East can lead a spade to set the contract, but that is an illusion.
North calls for the spade ace, then plays the ace and queen of hearts immediately.
East must cover, and declarer ruffs. Declarer then leads the diamond king, west winning. If he cashes the spade king, declarer can pitch two clubs on major-suit winners in hand. And if he does not cash the spade king, the spade winner vanished on the heart jack. In either case, declarer is able to get rid of one of his black suit losers.