Source: Slam-oriented auctions: Gadgets and Gizmos 
This brings us to one of the more important patches that 2/1 requires. When in my youth I first read about these methods, Edgar Kaplan remarked that the problem with game-forcing sequences was that neither hand knew when or how to limit or sign off. An action such as:
often leaves the next hand to bid in a quandary. They may have a slam drive or a sign-off; those hands present no problem. But the difference between a hand willing to initiate a slam-try, and one merely willing to co-operate in a same-try is vital. In our second example auction, into which category do you place A-J-xx-x Q-x-x-x A-x K-x?
If you cuebid, partner may use Keycard with K-x J-x-x-x K-Q A-Q-x-x-x, assuming three keycards will suffice to make slam playable. But the five-level is highly dangerous here. If you sign off over 3, would partner be out of line to give up when holding x A-K-x-x x-x-x A-Q-J-x-x? The secret is to have the following agreement. In uncontested actions, when an unequivocal eight-card majorsuit trump fit has been established, 3NT is never to play. Depending on which of the following two agreements you have (I prefer the former, Eric Rodwell the latter – make of that what you will!):
3NT is a non-serious slam-try, ready to cooperate in a slam-try but not to initiate one. All cuebids above 3NT are serious slam-tries, showing real extras in context. OR
3NT is a serious slam-try, showing real extras in context. Cuebids above 3NT are non-serious slam-tries, ready to cooperate in a slam-try but not to initiate one. My view is that the former auction gives less away. Using non-serious 3NT, in our examples above, opener’s 3 over 3 neither promises nor denies extras. If opener does bid 3 responder will bid 3NT and opener will sign off in 4. In the second example over 3-4-4-keycard will see the partnership reach the small slam.
It is important to agree that the convention only applies in non-competitive auctions (opponents’ doubles don’t count) and that the eight-card fit must be established. To my mind.
does not establish a combined eight trumps. You could argue that.
DOES show three-card fit since responder had an economical temporizing call of 3 available. But in the first auction 3NT would be a suggestion of a place to play. For example:
sets spades as trumps. But does 
guarantee an eight-card heart fit? Or could the two hands be:
You make the call.