Key bid can lead to excellent small slam

Playing with a stranger online is both demanding and exciting. Faced with this bidding problem, what would you bid with your regular partner?

Bidding
Dealer: North
Game all

North’s game-forcing 2C opener was a little thin, but just fine. The more normal rebid over my 2D negative relay would be 3H. Having opened 2C, a jump rebid indicates a solid suit, not requiring support from partner. 

Here, over a 2H rebid, South held support but a very weak hand, lacking key-cards, and so he considered bidding 4H. But, is South’s hand that weak? K♠ could well be a trick and his singleton club could provide one to three ruffs in dummy. So, South decided to jump to 4C. Would his partner understand the meaning of such a bid?

Any jump in a suit at a level higher than would have been natural and forcing — as 3C would have been — must be artificial, and the only logical interpretation here would be that South is agreeing hearts and indicating a singleton or void in clubs. There are alternate, expert, possibilities, but those you would certainly discuss in advance.

At the table, there was a long pause, followed by Roman Key-card Blackwood, a 5C response and a jump to 6H. North had trusted his strange new partner, and interpreted the bid correctly. Only one other pair managed to reach 6H; a regular partnership adopting similar means.