Source: [ilink url=”http://www.acbl.org/”]2005 ACBL Bulletins[/ilink]
Against Slam
When capable defenders choose a lead, they review the bidding, visualize the dummy and try to anticipate the play. Against a suit slam, leading the ace of an unbid suit is usually safe (and may be necessary to stop an overtrick at matchpoints).
Trump leads are uncommon. Against 6NT, a passive lead is usually best since leading from an honor may be costly when declarer’s side has almost nil the missing honors.
As South, decide what type of defense 10 conduct active, passive or trump-leading.
When the opponents bid this strongly, they will surely have the material for 12 tricks, and the defenders’ only chance is to win two first. Since aggression is in order, lead a low diamond.
If declarer misguesses and plays low from dummy, down he goes. On a passive spade lead, declarer can win in his hand, take the 
A, ruff a heart high and lead a trump. He has enough dummy entries to set up the long heart for his 12th trick.
Lead a diamond. You’re entitled to draw an inference from North’s failure to double either of West’s Blackwood responses of 5 and 6
.
Only a diamond opening lead has a chance to sink the slam.
Against 3NT you would lead a spade, but a spade lead is too dangerous against 6NT when East/West had a point count auction and North probably has nothing. Lead a passive diamond. Since North may play no part in the defense, you may
lead a deceptive 
10.
A passive lead gives declarer the opportunity to go down several wrong paths. A spade lead makes things easier for him.
Since dummy advertised a club suit, and East- West had a strong auction, an aggressive lead is best. You have too many points to hope North has the K Q, so you must try a spade. Since West’s 4
cuebid places the ace in dummy, lead the K even if declarer has the queen, you may gain by killing dummy’s entry to the clubs.
Lead the K and make the newspapers. No other lead defeats the contract.