This is an interesting hand at duplicate.   Playing Acol and transfers, the bidding will probably look like this

South    West     North    East

 1NT      pass      2H       pass

  2S       pass      3D       pass
  3S       pass      4S      all pass 

2H is a transfer to spades and 3D shows a second suit, minimum 5 - 4.  3S shows 3 spades and not a doubleton and North with his singleton club prefers 4S to 3NT.

The lead is likely to be the diamond 4.  As soon as dummy is tabled South can see that those playing 3NT will do well as the singleton club is facing a suit with 3 honours.   That's a pity so we must try to maximize our tricks in spades.   

The lead of diamond 4 is likely to be from the doubleton 43, but could be from 1064.   It looks like the diamonds will split 3 - 2.   If they are 4 - 1 it will be very difficult stopping a diamond ruff so let us assume 3 - 2 and proceed.  

You can see a losing club and at least one losing trump.  There is also a losing heart and the possibility of having a third round of diamonds ruffed.   If trumps are drawn early there will be no ruff and 10 tricks should be possible but can we find the 11th trick.   The losing heart can go on an established club after the ace is driven out and that will reduce us to two losers.   

Before you play trick one you must mentally play the hand.    OK, we need to drive out the trumps quickly and the best play is undoubtedly to play for split honours and a 3- 2 split.  So let the lead come to KD and then play a trump, the 10 is a good choice as it might tempt the Queen or King from West and solve that problem.  We assume this loses to East who will probably return a diamond but you win in dummy, come back to hand with Ace of hearts and play a trump finessing  the jack which you expect to win.  You then play the ace and drop the outstanding honour.   There will now be no diamond ruff and you can play the Jack of clubs, overtaking if necessary and driving out the ace.  WAIT, this plan fails as you have established the clubs but have no entry back to your hand to enjoy the clubs.  

Plan B.   Drive out the ace of clubs earlier, but if you do this they have time to play a second diamond when in with the ace of clubs and a third diamond (ruffed) when in with the first losing trump finesse if that hand holds the third diamond.  This doesn't work either.

Solution.  Good job you thought this hand through before playing at trick 1.   You need an extra entry to hand and it can only come from the 4th diamond.    You play the Jack of diamonds at trick 1 and overtake it with your King.   Now, follow your first plan.  Now when you drive out the ace of clubs and dummy regains the lead with the King of hearts you can play the diamond 5 across to the 8 and enjoy the established clubs which permit the losing heart to be discarded.   Thus the all important overtrick is achieved. David Telfer did extremely well to find this play at the table.

Note that in 3NT, you probably get a club lead, which you win in hand for the spade finesse. East wins and you win the club return. Now, you have a decision to make - take your 9 tricks, or take another spade finesse, and risk going down. Since it seems that 4S will make, possibly with an overtrick, the second spade finesse appears to be the better choice.