Cuebidding: You have seen cuebids used to show a limit raise, to ask if the overcall was a full opener, to ask for a stopper, and just to say “tell me more, partner.” Furthermore, a direct cuebid shows a playable two-suiter.
Here, the cuebids show first round control on the way to a slam in a known major fit. Cuebidding the same suit a second time shows second round control. Failing to cuebid a suit denies the control.
The bidding: Even without a spade honour, South opens one spade and partner bids a major game raise, two notrump.
Two notrump shows four card major support, enough values for game and any distribution. A splinter is not appropriate here because it gets the bidding too high. The bidding space that a splinter jumps over is very valuable for cuebidding.
Opener then bids 3 showing a singleton club or a club void. The singleton club may be the ace but no other honour.
North then cuebids hearts. His three heart bid shows first round control. South knows this to be a void because he has the ace himself. South then cuebids first round control of clubs.
North recognizes that he has no wasted club values. He could bid four spades at this point, but his hand is too strong. Therefore, he cuebids hearts again showing second round control too. South knows a void is also second round control; consequently, he realizes North is temporizing so that South can use keycard Blackwood.
South obliges and bids four notrump asking for keycards. North could respond five notrump which conventionally shows two keycards and a useful void but cuebidding has already made that very clear. North bids five hearts showing two keycards and no queen of trump. South bids his vulnerable grand.
The contract: Seven Spades by South
The opening lead: The eight of clubs. This is a safe lead of a club to what is likely South’s singleton Ace. South used keycard Blackwood which indicated that he has no void and no worthless doubleton.
The play: South wins the ace of clubs, draws two rounds of trump and claims.