East plays in 4S, after North opened with a weak 2H (6-9 points and 6 hearts). 

South leads the HJ, to HK and HA. (The HK is played to preserve the HQ as an entry to hand.) 
North returns a trump which Declarer wins.

Declarer can see a heart and a club loser so must keep to 1 diamond loser if he is to make his contract. He cashes the HQ, eliminating hearts, and leads the C6, hoping to sneak a club trick and avoid a club loser.

But South wins the CA and returns a trump. Declarer wins in Dummy, and now knows that North had 2 spades and 6 hearts to the ace, and has so far shown 5 points. Also, all trumps have been drawn. So he can cash 2 top clubs and lead the last one, to check how many clubs North has.

North plays the CJ on the third round and discards a heart on the fourth round, which Declarer ruffs. So North has shown he has only 2 diamonds and has shown 6 points so far. North is unlikely to have the DA (giving him 10 points), and being vulnerable and with a 6-3-2-2 distribution, is unlikely not to have the DQ (without which he would have only 6 points).

So Declarer assumes North has DQx. So a diamond to the DK and a small diamond back will defeat the defence,

Either North wins and gives a ruff and discard or South overtakes the DQ with the DA and has to concede the last 2 tricks.