Early on, new bridge players learning to defend are taught to respect the maxim "2nd Hand Low", recognizing that saving high cards and routinely following with a low card when in 2nd seat is often the correct play. But whether defending or declaring, this guideline must sometimes give way to other tactics to bring a contract home.
When dummy hit, the position of the Ace of Clubs was unknown, and might be with west, despite east's spade overcall. However the bidding and the opening lead did mark the east hand for both the ace and jack of spades, and east with a very likely singleton or doubleton.
On the lie of the cards, if south had played small from dummy, allowing the ♠8 to run around to his king, only eight total winners would have been scored - one spade, two hearts and five diamonds - before south would be forced to lead clubs. West would then pounce with his club ace to lead the ♠4 through dummy, trapping the Q-10 and allowing the defense to score four spade tricks and the ♣A for down one.
Well then, how about the ten of a clubs from dummy at trick one? Not good enough, since east would cover as cheaply as possible. At that point south could dare not duck, for fear that east, if holding the ace of clubs, would play ace and another spade, still holding south to at most eight tricks.
South guards against a wrong-sided ace of clubs by putting up the ♠Q at trick one - the hand's key play. East gains nothing by ducking, and if east wins, spade suit leads are effectively frozen, since continuing spades from east would make both the ten and king winners. South has gained a tempo that allows enough time to establish a ninth trick in clubs, regardless of the position of club ace.
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