This approach was introduced as an option for responder at intermediate or beyond.
In adopting this approach you will have released an immediate response at the three-level for an alternative purpose. You should at the same time have incorporated a method such as non-promissory Stayman or Sharples-Marx transfers to show the invitational raise to 2NT.
Partner has either bid 2♠ promising at least six clubs, or 2NT promising at least six diamonds (some agreements permit just five cards in the suit in certain sequences).
A majority of tournament players will combine this approach with two-tier responses.
This is a powerful method.
The most important application of the method is to test the solidity of a suit, which will on occasion enable you to bid 3NT based on the running minor suit when your combined holding of high-card-points is a little light.
The constructive sequences for responder (particularly five minor followed by four major, forcing - and a raise to four setting the suit for a minor-suit slam-try) are available just as they were with simple four-suit transfers, and as a bonus it also provides a method of finding a weak take-out at the three-level on a hand which is 5-5 in the minor suits.
However, these considerations are in the mind of responder on his first bid and on his rebid.
Your responsibility as opener is merely to respond to the transfer in the agreed manner.
The usual system of two-tier responses is:-
step 1 any hand which does not qualify for a step 2 response;
step 2 any hand containing Qxx or better in the implied minor.
A J 7 5
K Q 8 4
T 8 6
Partner has transferred to clubs.
denying three cards to a top honour in clubs.
A J 7 5
K T 8 4
Q 6 4
Partner has transferred to diamonds.
promising at least three cards to the queen in diamonds.
It should be noted that some pairs permit a shorter holding to count as suit support - eg Ax or Kx, as well as Qxx,
and some authorities include any fourteen point hand in the positive step 2 response.
My recommendation is that you stick with a minimum of Qxx as above. The critical factor is to ensure that you have an entry to the running suit in dummy, and the third card will frequently be essential in this respect as a defender holds up for two rounds.
Some pairs reverse the meanings of the two-tier responses.
This is inferior because it disables the final advantage of using these transfer. With 5-5 in the minors in a weak hand, responder can bid 2NT promising a transfer to diamonds. If partner accepts the transfer with a bid of 3♦ then he can pass, in the knowledge that he will be playing in at least a 5-3 fit.
If he declines the transfer with a bid of 3♣, then he is likely to hold three cards in clubs, and you will again be happy to pass.
For continuations after a two-tier minor suit transfer follow one of:-