Any user of Roman Keycard Blackwood (RKBW) must have at some time been frustrated that he cannot bid RKBW because a response would take the bidding too high and force him into a slam missing two aces.
The solution is Kickback; a form of Blackwood that guarantees you can always sign off at the five-level.
Bid and Responses Bidding the suit above the agreed trump suit at the 4 level is Roman Keycard Blackwood. The responses are the same as for RKBW, just several bids further down.
For example, in the following auction: 1C Pass 1D Pass 4H Pass 5C Pass 5D All Pass 4H is RKBW, 5C shows 2 or 5 Keycards without the Queen of Trumps and 5D is a sign off, as from his hand he knows he is missing two Keycards. Using normal RKBW, a bid of 4NT by opener would have given a response of 5H, which would have forced the partnership into a slam missing 2 Aces. This perfectly illustrates the usefulness of Kickback.
Notes One disadvantage of Kickback is that it means that some bids are unavailable as cuebids. For example, in the auction: 1S Pass 3C Pass 4C Pass 4D 4D would normally be a cuebid, showing an Ace or Void in Diamonds. However, playing Kickback it would be RKBW. One solution is to play that 4NT is the equivalent of a cuebid in the Kickback suit.
It is very important to agree when a bid is RKBW and when it is natural. For example: 1S Pass 2C Pass 4H Pass 4S. Is 4S RKBW or natural? I personally play that if there is not agreed trump suit, bidding a previously bid suit is natural.