The two way checkback is a convention widely used by the world's best pairs in the sequences:

1x 1y             

1z

 

It consists in using two RELAYS.

One to 2♣ for limit hands (or weak in a minor) . The other to  2♦ forcing game.

On the 2♣ RELAY  opener rebids normally 2♦ and responder reveals his hand (pass or 3♣ is weak)) .

On the 2♦ RELAY opener describes.

I would approach it under the MAJEURE SECURISEE and in the following sequences:

 

 

 

 

 

Indeed, in other sequences opener has already responded to the main problem of responder : namely, knowledge of major 5-3 fit and the zoning of the 4th fit. And after opening 1♦ when the opener rebids 1NT GAZZILLI he has then become the captain of the sequence.

 

The 2♣ RELAY  is usually done with 5+♥ or 4♠ in a limited hand in sequence 1.

However, in the sequence 2, as the opening is unbalanced natural  it will often also with a weak hand and a preference to ♦ .

In the sequence 3, we can respond 1♠ with a good 5+ card suit and  3 cards support in heart in a hand of 8-11 HCP.

For example respond 1♠ with : ♠ AD9872     ♥ V54    ♦ R5    ♣  105. Indeed,  if opener rebids 1NT, it is possible thanks to the two way checkback to set the level of the contract while showing 5+ spades. If the opener rebids the two level, it shows 14+ HCP and you can bid the game.

 

One may wonder what to do with a limit long minor suit? For consistency, we make a direct jump to the 3 level after the response of  1♠ to show the hand as it is done in all other sequences of style       1m       1x

                                                                                         1y

However, a weak hand with a long minor  is relayed to 2 ♣.

Which gives :

 

2♣ contains usually limit hands with 5+♥ or 4♠ but it can be weak hands with long minor. The limit 6m-4 ♥ is deal with an immediate jump to 3-level in response to 1 ♠.

 

 

The immediate response of 3♠ (on 1 ♠) shows  5♥-5♠ 8-9 HCP.

 

 

2♦   = normal rebid by the opener.

          → 2♥ = 5+ cards                                             → 3♥ = good 6th suit 11 HCP, rather misfit in ♣.

          → 2♠ = 11-12 DH , 4 cards support             → 3♠ = 13-14 DH , 4 cards support   

          → 3♦ = 4♥-6♦ 11 HCP, with 9-10 HCP direct jump to 3♦ on 1♠.

         → 2NT = 11 HCP exactly, with 12 HCP direct jump to 2NT on 1♠.   

         → 3♣ =Warning : shows a weak hand.

2♥   = Type 5♣-4 ♠-3♥-1, zone 3 (in zone 4 the opener would have bid 2♠ the first round) .

2♠  = 6♣-5♠.

2NT = Balanced, 18-20 HCP.

3♣   = 6+♣-4♠, too misfit for 2♦.

3♦    = type 5♣-4♠-3♦-1 too strong for  2♦ .

 

 

 

 

 

2♣ contains usually limit hands with 5+♥ or a diamond fit 10-11 HCP, but it can be a weak 6♣-4♥.

The limit 6♣-4♥ is deal with an immediate jump to 3 level in response to 1 ♠.

 The immediate response of 3♦ (on 1 ♠) shows a 4+card support in a hand of 7-9 HCP (with 10-11 HCP bid 2♣ first).

 

2 ♦   = normal rebid by the opener. (Zone 1 or 2)          

            Warning: If the responder rebids 3 ♣ he shows a weak hand .

Others = description of good hand zone 2-3. (in zone 4  the opener would have rebid  2♠ or 1NT GAZZILLI in the 1st round).