The idea is to strip the hand, trying to avoid two spade losers if the clubs don’t come in for three tricks – West having a doubleton ♣Q J,
♣Q 9 or ♣J 9. The latter two holdings allow for a ruffing finesse, setting up a club in dummy for a spade discard.
Win the ♥K, cash the ♦K Q, the ♣A K, and if nothing desired happens, cross to the ♥A, ruff a club and lead the ♠J.
You are playing East for the ♠10 x, twice as likely as East having the ♠Q 10 or ♠K 10. (It is assumed that had you led a spade to the ace at anytime, East would unblock with K–x or Q–x). After West covers, win the ace, ruff a club, and lead a spade towards dummy’s 9–4. End of story. East, upon winning the ♠10, is forced to concede a ruff and sluff. Had East unblocked the ♠10 under the ace, ruff a club and lead a spade up to the 9, again losing only one spade trick.
Thanks to Tim Bourke of Australia for yet another of his neat slam-hand creations.