In a cuebidding sequence (in my partnerships, anyway), both sides are in a cooperative mode; they're mutually exchanging information until someone knows enough to set the contract or at least give up on more, at least from his view. In contrast, when someone bids keycard either earlier in an auction or by breaking off a cuebidding exchange, that partner is taking the lead role and essentially putting himself in the best position to judge what's going on, if he's given information (or in other words, he's assuming captaincy). Partner's either asking or telling about spades in an effort to get to something above and beyond 6♦; per the fact that he's assumed control by bidding RKC, he should be asking. My spades are worthless, so I'm bidding 6♦.