[Trick13]

If we have an auction where partner bids 3NT, suggestion to play, and we pull it to 4m, then optional keycard, where the first step shows a lack of enthusiasm for slam, seems good. Control bidding might be good too, but there isn't usually room for both.

Imagine a different auction where we are in a game force and maybe we had suit agreement at 3m, there may have been some 3 level cues which could have been NT probes, but 3NT was by-passed for 4m, or perhaps we didn't reach suit agreement until 4m. In these cases it is quite likely that only one hand would know what to do after hearing a keycard response, and the flow of the auction may have caused the wrong hand to bid 4m. In these auctions I think we should use the first step, 4m+1, to ask partner for his/her keycards, and for the other bids to show keycards and by implication less slam interest.

While pondering this, I saw this hand on BBO which illustrates the idea.

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You might not get suit agreement until South bids 4♣. North then bids 4♦ asking for South's keycards. If 4♣ was minorwood or optional keycard, South would not know how to proceed after hearing two keycards.

[Kungsgeten]

We play something similar to this. If an "unlimited" hand bids 4m it is ORKC, but if a limited hand does it (usually to confirm a trump fit, or maybe because lacking stoppers) then it is natural and the next step asks keycards.