Kickback Convention
If you've ever had a Roman Keycard Blackwood disaster, as many of us have, it might have been because the prescribed responses would take the bidding above the place where you would like to stop, and force you to bid a slam off two aces.
NOTE: The RKCB 5
shows two keycards with the trump queen, which means if your suit is not spades,
you are committed to the 6-level. Some experts refuse to use this part of RKCB unless the trump suit is spades, because
they believe it's too dangerous for the other suits.
Kickback was developed as a way to avoid bidding a slam missing two aces. It's a modification to Roman Keycard Blackwood that allows the partnership to sign off at the five-level.
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If you bid the suit above the agreed trump suit at the four level you are starting Roman Keycard Blackwood
at a lower level. The responses are the same as for standard RKCB, but this adjustment will allow you to
stop sooner.
If you suit is spades, then Kickback and other Blackwoods are the same -- 4NT is the asking bid.
When hearts are the trump suit then 4
is the asking bid.
When diamonds are the trump suit then 4
is the asking bid.
When clubs are the trump suit then 4
is the asking bid.
There is a very good description of Kickback here... bridgewebs
Roy Wilson