Source: ABF Newsletter Sept2015
Kickback was first introduced by Jeff Rubens in a series of articles in the US Bridge World in 1980-81. In Kickback, the Key Card ask is the suit above the trump suit at the four level: 4
is Keycard for clubs, 4
is Keycard for diamonds, 4
is Keycard for hearts and 4NT is Keycard for spades (as normal).
All these sequences are Kickback:
One advantage of Kickback is that the space saved may allow the partnership to bid a good slam:
1. Kickback for hearts
2. 2 Key Cards + Q of trumps
3. Asking for specific kings
4. K
In normal Key Card, 5NT after 4NT by the asker shows all the Key Cards and the trump queen are held, and asks for specific kings. On this deal, over 5NT the responder would not be able to show K without going above 6, which might take the partnership too high.
In Kickback the asker repeats the Kickback suit to ask for specific kings.
The extra step gained by Kickback allows opener to show the king above the trump suit (the Kickback suit) with 5NT. This would make all the difference if responder held instead A74, KJ1042, A3, 
Q74.
On this hand you need opener to hold K and not K so it is dangerous to bid 5NT in case opener bids 6 and not 6. In Kickback you can repeat ask with 5 on both responding hands.
Sometimes the Kickback suit would have a natural meaning so you need to bid one step higher:
In both these sequences responder’s 4 would have the natural meaning of showing hearts, so Kickback would be 4:
Then again you could agree that the jump to 4 is Kickback in both auctions, and bid 3 then 4 on both to get to 4 . When a minor is agreed in Kickback the partnership is able to sign off in 4NT:
1. Strong preference but not forcing
2. Kickback
3. 1 Key Card
4. Signoff
The Kickback book I refer to (Kickback, Slam Bidding at Bridge by Robert Munger, $14.95) prefers 0-3-1-4 as the responses to Kickback, so 4 shows 1 Key Card and opener signs off in 4NT. To ask for specific kings, responder would continue with 5 , the original Kickback suit.
Kickback takes a bit of getting used to, and there are quite a few new rules to learn. Also, you need to double check when you make a bid. At the recent Coffs Congress I held AK97652, 3, A1072, 9, my LHO opened 1 and partner overcalled 1 , so without a care in the world I bid 4.
Yipes, when partner started to think I had the sinking feeling that partner had taken my 4 as Kickback. Partner replied 5, 1 Key Card, and I bid 5. Partner passed, but spades were 5-0 and 5 failed by a trick.
We later agreed that leaps to game like this were to play. Even so, 1 by me would have been forcing over partner’s 1, overcall so next time I will bid 1.
You can expect quite a few hiccups and a few disasters, but Kickback will add a significant edge to your game with the extra room to ask for specific kings and even third round controls. Plus it’s a lot of fun.