North-South vulnerable.  
South deals.
                              North
                              S: A K J 10 3       
                              H: A Q J 10 5  
                              D: K 7         
                              C: J       
            West                             East
            S: Void                          S: 9 7 5 4      
            H: 8 7 3                         H: K 9 4 2    
            D: Q J 10 6 5 2                  D: 9                 
            C: K 10 8 3                      C: Q 7 6 4     
                              South                         
                              S: Q 8 6 2
                              H: 6          
                              D: A 8 4 3   
                              C: A 9 5 2  
          The Bidding:  
                  South    West    North    East                  
                   Pass     2D      Dbl     Pass
                    3S     Pass     4D      Pass
                    4S     Pass     4NT     Pass
                    5S     Pass     6S      Pass
                   Pass    Pass
          Opening lead: QD
     This hand is from a local duplicate and was surprising to me
in that only one pair in the field of ten bid six spades and made
seven.  Two pairs bid and made six.  The rest of the field bid
only the game save for one pair that was down one.
     There is no good reason not to bid a slam on these cards. 
If West opts to pass at his first turn, North should open two
clubs in spite of the fact that he is two-suited.  He has a four
loser hand that can play in either major suit.
     After the pre-emptive diamond bid, North starts with a
double.  South responds three spades to show that he has at least
four spades and at least eight points.  North's four diamonds is
a cue bid showing a very big hand.  South could bid five clubs to
show his lowest ace but I think four spades is a reasonable bid. 
If North wants to look for slam he can bid the Blackwood
convention, which is what he does.  
     The pair are playing Roman Keycard Blackwood and five spades
shows two aces and the queen of spades.  This means that South
has at least ten points.  If he had the king of hearts he would
have opened the bidding, so it is a safe bet that he does not
have that card.  For this reason, six spades should be the final
contract.
     On the lead of the queen of diamonds, declarer plays low
from dummy in case East can trump this trick.  East follows suit
and declarer wins the ace.  A small spade to the ace reveals the
bad trump split.  Declarer should return to hand and take the
heart finesse.  It will lose and he will only make six, but look
at what could happen if he instead plays the ace of hearts and
then takes a ruffing finesse by leading the queen from dummy. 
East plays low and declarer discards a diamond or a club.  If
West has the king of hearts he will win and play another diamond
for East to ruff.  This is too risky.  If the finesse fails,
there is no lead that East can make to endanger the contract.
     Even with no bidding by the opposition, once the trump split
is known, make the play that will force the defender with the
trumps to make a lead if the play does not work.  This is always
the safest play in this situation.
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