This convention is used after a natural 1C opening. This method is used because major suits are important and it can be difficult to show them later in the bidding if you haven’t a good hand. Therefore the Walsh convention bid 1H/1S instead of 1D every time the responder has no GF hand and at least 4+ in a major suit.
1C - 1D: 4+Diamond. 6+Diamond if 6-12 HCP.
1H: 6+HCP. 4+Heart, may have a longer diamond suit.
1S: 6+HCP. 4+Spades, may have a longer diamond suit.
1N: 6-10 HCP. May have 4/5 Diamonds.
!C - 1D
1H : 5+Club – 4 Hearts.
1S : 5+Club – 4 Spades.
1NT : 12-14 HCP. Balanced hand. May have 4 card major.
Remember that if the responder bids 1D, all further bids will show a GF hand except 2D/3D who show a 6+Diamond hand and under 13 HCP.