This slam was well bid by Iain Sime and John Matheson in the first 2012 Camrose match. When East opened a strong no-trump West saw the possibility of slam, but not the certainty: there might be a shortage of controls, or the club suit might not run. He started by transferring into clubs, but was neither surprised nor downhearted when partner did not break the transfer to show positive club support. 4D was Kickback Blackwood, where the suit above the agreed trump suit asks for aces. This space-saving manoeuvre, particularly useful when your agreed suit is a minor, allowed him to discover that partner had three aces. And there was still room to ask about the queen of trump. When John showed that card Iain could count 12 tricks, and signed off in 6NT, thus avoiding the danger of a diamond ruff.
Had John shown just two aces Iain would sign off in 5C, something that is rarely possible if we use 4NT as the ace-asking bid.