The convention was designed to use with Neapolitan cue-bid (showing a first or second round control after a suit fit has been established) and it allows to the bidder to clarify to his partner how many Aces/key card (on 5 Aces like RKC) he has. In RKC the five aces are the four aces plus the king of trump.

The basic idea is that in the development of bidding, after that a cue-bid was made, declarer bids 4NT to show an even number of Aces/key cards (0, 2 or 4) and nothing other (not cue-bid on suit of last partner's cue-bid), if he bids something higher than 4NT that bid shows an odd number of Aces/key cards (1,3 or 5) and a first round control of that suit. In other words, 4NT is not an asking bid, but a showng bid.
Some examples:

Ptr Opp Ptr Opp
1H Pass 3H* Pass
4C** Pass 4H*** Pass

in this action:

* is a limit raise

** cue-bid in club (first or second round)

*** No cue bid in diamond

**** Turbo: the opener has even Key-Cards (0,2, or 4) in this action 2 or 4 with diamond cue-bid

Possible hands can be:
Opener: S xx H AKJ xx D A C Axxxx

S Axx H Txxx D QJxx C Kx or
S QJxx H Qxxx D QJx C Kx or
S KQx H Qxxx D xxxx C Kx

Note that: with responder Hand 1 slam is a good chance, with Hand 2 slam has no chance, and with Hand 3 slam is almost 100%.

Ptr Opp Ptr Opp

1C Pass 1S Pass
3S* Pass 4D** Pass
4H*** Pass 5D****

in this action:

* is strong hand support

** cue-bid (I or II round) in diamond and no cue-bid in club

*** cue-bid in heart and implicit cue-bid in club

**** Cue-bid in diamond that shows: absolute control in diamond (I and
II round), odd number of 5 Aces (1-3 or 5), real good hand (with less interest could bid 4S), not Queen club (he shows xx or xxx in club).
Possible hands can be:

Opener: S AJxx H Ax D xx C AKxxx

S Qxxxx H Kxx D AKx C xx or
S xxxxx H Kx D AKx C xxx or
S Kxxxxx H Kxxx D KQxx C xx

Note that: with responder Hand 1 slam is a good chance, with Hand 2 have no chance, and with Hand 3 is almost 100%.

Good agreements to use Turbo are:

- splinters is not enough to use 4NT as Turbo (so 1S - 4D= raise to
spade and splinter in diamond, 4NT is Blackwood; but 1S - 4C (splinter in club) - 4D - 4S - 4NT, this 4NT is Turbo and implicit cue-bid in heart);

- who overtakes manche level, show more than minimum number of Aces (at least 2 of 5);

- use cue-bid only in clear suit is supported, on implicit suit can be used only by real good partnership;
- don't abused with cue-bid: you clarify your hands at opponents too, and they can use "double" for lead.

I'd like to thank stefano, tefano@infopubblica.com, for submitting this convention, along wth several very helpful sample hands and auctions.