RKCB is a version of the standard Blackwood 4NT that’s used to ask partner how many aces he holds. RKCB improves on this convention by allowing you to check on five important cards – the four aces and the king of your trump suit.
RKCB is “on” (asks about the five key cards) after you and partner have agreed on a trump suit. To do that, your auction must have confirmed support with a raise or one of you must have shown great length in a suit. This is one of the most difficult aspects of RKCB to handle, so it's important for a partnership to discuss what types of auctions will confirm a trump suit.
4NT can also be RKCB for the last-bid suit. It must be a natural suit bid. For example, after 1D by partner, 1H by you, 3C by partner, your jump to 4NT is RKCB and it's implied that clubs is your agreed suit.
If you bid 4NT directly over a 1-level opening bid (1H by partner - 4NT by you), it should be "regular" (aces-only) Blackwood. No suit has been raised, so opener's response shows only the number of aces.
If you have an agreed trump suit, the responses to 4NT give information about five key cards -- the four aces, plus the king of the agreed suit. The meanings of responses to 4NT are:
5C -- 0 or 3 key cards
5D -- 1 or 4 key cards
5H -- 2 key cards, no queen of trumps
5S -- 2 key cards with the queen of trumps
5NT -- 2 or 4 key cards and a useful void
6 of a non-trump suit -- 1 or 3 key cards and a void in that suit (this is only used if the void suit can be shown below 6 of your trump suit)
The responses above are standard and are commonly called "30-14". A popular modification is to flip the meanings of the 5C and 5D responses, which makes it "14-30" (5C shows 1 or 4 keycards, 5D shows 3 or 0).
After a 5C, 5D, 5H or 5S response, the Blackwooder can continue with 5NT to ask about the number of kings you hold. Since you've already shown (or denied) the trump king, you don't count this when you make your 6-level response to the king-ask. The steps are 6C (no outside kings), 6D (1 king), 6H (2 kings), 6S (3 kings).
Many pairs modify this and reply to the king-ask by showing a specific king instead of the number of kings. With this agreement, after the 5NT ask, you bid the cheapest suit where you hold a king -- a response of 6D shows the diamond king and denies holding the club king. With no kings, you bid six of your trump suit.
You should also retreat to six of your trump suit if your cheapest king is higher in rank than the trump suit -- for example, your agreed suit is hearts and your only king is in spades.
If the Blackwooder uses the 5NT king-ask, he guarantees that your side has all five key cards and he signals that he's interested in a grand slam. If you have significant extra strength or an undisclosed source of tricks (a solid side suit, for example), you can accept partner’s grand-slam try immediately (without answering with information about your kings) by jumping to 7 of your suit.
If responder has 2 key cards, his bid of 5H or 5S will also tell you if he holds the trump queen. You can also get this information after a 5C or 5D response. To do this, the Blackwooder bids the cheapest step (5D over a 5C response, 5H over 5D).
If the Blackwood bidder uses this step to ask and:
You do not hold the trump queen -- Retreat to 5 of your agreed suit.
You hold the trump queen but no outside kings -- Bid 5NT.
After you bid your cheapest king, if the Blackwooder has room (and is interested in a grand slam), he can bid a new suit to ask if you have a king in that suit, too. The Blackwooder does not show his specific kings.
Here's an example auction:
♠AK983 ♠Q76
Opener: Responder:
1S 2NT (forcing spade raise)
4NT (How many key cards?) 5D (I have 1 or 4 key cards for a spade slam.)
5H (Do you have the ♠Q
?) 6D (I
have the spade queen AND the diamond
king, but I don't have the club king.)
6H (I need the heart king.) 7S (I have the heart king, too.)
Pass (Yay!)
NOTE: As mentioned above, there are several variations of RKCB responses. You should go over these parts of RKCB with your partner to be sure you're both playing it the same way.