When a major suit has been agreed upon, a jump to five of that suit is a small slam force.
When opponents have bid a suit, it asks partner to bid slam with first or second round control in that suit.
When the opponents have not bid a suit, it asks partner to bid slam with first or second round control in the unbid suit.
When opponents, have not bid a suit, there is no unbid suit or cuebidding has shown controls in the opponent’s or unbid suit, then a jump to five of the major suit, asks partner to bid slam with two of the top three honours, otherwise pass.
The bidding:
South opens One Spade and West pre-empts to Three Clubs. North’s bid of Four Clubs is not a control-showing cuebid. It simply tells partner, that Spades are trump, and he has a limit raise or better.
South jumps to Five Spades, asking North for a Club control. North has one and bids Six Spades.
