Bidding conventions are evaluated based on the amount of information they allow the partnership to exchange against the natural bids that become unavailable as a result of the utilization of the convention. This hand illustrates the use of Roman Key Card Blackwood, which every competitive, tournament partnership should be using.
South opened the bidding with one club and then reversed to two diamonds after partner's response. This sequence shows a very strong hand and confirms that the club bid was not a convenient opening but showed a real suit. North's four club bid shows good club support and denies a heart stopper.
South's hand will be much stronger if he can find out if partner has the ace of spades. Four no trump would be "Blackwood" in most standard partnerships, but here they were playing "Roman Key Card Blackwood". Once a trump suit has been established, the king of the trump suit becomes a "Key Card" for the purposes of the responses to four no trump. There are now five key cards. A five club response shows zero or three key cards; a five diamond response shows one or four key cards (variations exist depending on what reference you are using); five hearts shows two key cards without the queen of trump; and five spades shows two key cards with the queen of trump.
Based on the cards held by the four no trump bidder and the previous bidding, it should be fairly easy to pinpoint exactly which key cards are in partners hand. In our example, South already has four key cards so the five diamond response shows one key card, the ace of spades. South's five heart bid is now conventional and asks partner if he has the queen of trump. The five no trump response says that he has the queen, but no other kings. Since it is unlikely that North would have jumped to four clubs with only an ace and a queen, it is reasonable to assume that he has another queen somewhere, making seven clubs an odds on favorite.
Unfortunately, you don't see bridge hands with slam potential often enough. But when you do, you are not giving up any "natural" use of the bids involved by using this convention. The only requirement is to remember the convention when it comes up. This can be more difficult than it sounds when you are caught up in the excitement of bidding a grand slam.