Here lawrence advocates a 5♣ cue bid to try and figure out whether partner has a diamond control. His proposed auction continues 5♣-5♦-6♥. In the comments about 4NT, RKCB, he states "Oops, where's your diamond control?"
What I don't understand is, we have 4 keycards. So, why is blackwood not adequate to finding partner's diamond ace? Is the purpose to try and get to slam opposite a shortness control in diamonds? For that partner would have to have all the remaining non-diamond honors to justify his game forcing bid AND diamond shortness AND opponents didn't bid diamonds despite holding 10 cards in diamonds with AKQJ distributed between them. What am I missing?