With a trump suit agreed & cue-bidding engaged, 4N TURBO arrives!

The exceptions in which TURBO is NEVER used are:
Leading up to a TURBO bid (4N or above) the partnership exchange cue bids on an equal footing, then TURBO kicks in at 4NT or above.

The great advantage of TURBO is that one partner gets to know the partnerships key controls (RKCB+) as 4NT is bid or bypassed and cue bidding may continue!
Whereas with (E)RKCB key cards are only determined when a 5-level response to 4NT is given.
So, you're cue-bidding (see the rules!) and you're approaching the 4NT zone!
Just like with (E)RKCB, a TURBO 4NT is just about all Relevant  Key Cards (RKC) -The relevant aces & kings. Read on...

The essence of TURBO lies in the following statements:

  1. Actually bidding 4NT shows an EVEN (2 or 4) total number of  Key Cards (KC) - it also denies any further control cue-bid in any remaining bids under 4N!
  2. By passing 4NT shows an ODD total number of KC (3 or 5) which is also pinpoints a further control. Bids made beyond 4NT must be 1st round controls or 2nd round controls where a first round control has previously been expressed
  3. One more little rule: one must not pass 5 of the agreed suit or 4 of an agreed Major suit  without holding the  Queen of trumps! Simply bid 5 of the agreed suit to indicate lack of the Queen! or cue bid below it.
  4. One should not initiate cue bidding without at least 2 KC! or a KC and a useful void.
  5. Cue-bids may continue above the 5-level in the agreed suit showing any additional feature NOT bypassing 6 of the agreed suit.
  6. 4, 5 or 6 level bids in the agreed suit may be passed or bypassed by partner!
So in the 5rd rule above, without the Queen of trumps you need to bid the agreed suit - never cue bid above it. This bid is neither FORCING nor a command to end the bidding - that's entirely up to your partner. It simply signifies  not possessing the Queen of the agreed suit. If partner now continues after you bid game it neither shows or denies the Queen of trumps BUT it does show slam intent and he is aware his partner does not possess the Queen of Trumps.

TURBO takes a little effort to master, but i'm sure once it is well understood, it would always be the preferred  route to bid good slams (even thin ones)
and equally to avoid bad slamx! Except for clearcut cases where ERKCB is all that is required.. It also adds more clarity on whether to go on to a grand as cue bidding continues after 4N! Most importantly of all it efficiently uses bidding space more while simple Blackwood is wasteful.
It also allows for termination at the 5 level where Blackwood responses to 4N may commit to the 6-level!

There are 3 situations where a Spades¦Hearts fit & game values are agreed below 3N:
2Ma - 2NT (also 2
-2)- 3¦¦ -?  1X - 2¦ - 3¦  - ?  &  1NT - 3¦ - 2steps - ? these  sequences will have positive  responses and kick-off cue bidding  above 3NT. Now 3NT is TURBO showing an EVEN number of Key cards and highly unlikely to be rquired as a natural bid, while making a cue bid past 3NT shows an ODD number of key cards. This is a useful tool to judge the possible final level bidding level before bidding bypasses the game level.
 This is an advanced usage of TURBO at 3N - agreed by agreement.