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

The exception is that TURBO is NEVER used following a 2 opening bid.
In the 2 scenario we have the stronger hand interrogating the weaker from start to finish! - However leading up to TURBO the partnership exchange cue bids on an equal footing, then TURBO checks controls at 4NT. It would be a incorrect to mix cue-bidding leading to TURBO with other sequences leading which lead to 4NT ERKCB style or 4NT for Kings which specifically follows a 2C opening.

The great advantage of TURBO is that one partner gets to know the partnerships key controls (RKCB+) as 4NT is bid or passed by and cue bidding continues!
Whereas with (E)RKCB key cards are only determined when a 5-level response to 4NT is given and that would automatically discontinue cue-bidding.
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) number of Relevant Key Cards(RKC)
  2. A RKC may be a K of any suit opposite a disclosure of lacking a 1st or 2nd round control
  3. By passing 4NT shows an ODD number of RKC (3 or 5)
  4. One more little rule: one must not pass 5 of the agreed minor suit or 4 of an agreed Major suit  without holding the  Queen of trumps!
  5. One should not embark on cue bidding without at least 2 KC!
So in the third rule above, without the Queen of trumps you need to bid Game in the agreed suit - not cue bid above it. This bid is neither FORCING nor a command to end the bidding - that's entirely up to your partner.
When a minor is the agreed suit, when holding the Queen of trumps you may pass by the game level when (a) you're comfortable to continue to a slam (b) have a further control to announce. Therefore 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.
When the agreed suit is a major we are not passing game in the major without the Queen. This is quite a useful as it ascertains the Queen of trumps beforepassing game level.  This allows cue bidding to proceed right up to game without fear of getting too high when the Queen is vital. Nothing stops partner from progressing further towards a slam knowing the Queen of trumps is missing.
So we must emphasise that cue-bidding on this scale needs strict adherence to cue-bidding rules so that both partners have the same understanding of the auction.

TURBO takes a little effort to master, but i'm sure once it is well understood, it would always be the route to bid good slams (even thin ones) and equally to avoid bad slams. It also adds clarity on whether to go on to the grand! Most importantly of all it uses less bidding space then any Blackwood variant.

There are 3 situations where Spades¦Hearts fit & game values are agreed by the 3-level: (Classical TURBO does not cater for minor suit 3NT TURBO bids, however in Beta it is possible following  the  last sequence given below.)
2Ma - 2NT - 3
¦¦ -?  1X - 2¦ - positive - ?  &  1NT - 3¦ - 2steps - ? these 6 sequences will have positive  responses under 3NT. Now 3NT is TURBO showing an EVEN number of Key cards while making a cue bid past 3NT shows an ODD number of key cards. This is a useful tool to decide on the possible final level bidding bidding can reach.

Only in the case of 1NT - 3¦ where technically a 6-2 fit is already guaranteed (although it my only be a small doubleton), so we may consider the following: combining both turbo & trump request responses:- use 3NT directly with 0 or 2 KC and with 1 or 3 KC show the trump request responses directly with 3¦¦¦. Then after 3NT showing 0 or 2KC use 3 as a further relay to remake the trump request. Responses will then be 3¦¦¦NT.
After 1X - 2¦ - 2¦NT(negative responses) - then positive response for X: & TURBO