Source: 
https://www.larryco.com
Walsh is not really a convention, but more of a «Style.»
It deals with the responses to a 1
opening bid — and the follow-ups.
Responses to 1
Let’s start by examining the auction 1—Pass—1
. Should responder bid up the line (bidding 1 with 4 or more) or should he bypass diamonds? For example, what should he respond to 1 with each of these hands?
With 1), respond 1 and later show spades.
With 2) and 3), bypass the diamonds and show the major right away—responder might get only one chance to bid. This is the modern treatment (some call it «Walsh.»)
The theory is that with a «one-bid» hand you bypass diamonds so you can make sure to introduce your 4-card major into the auction. If you know you will get to bid twice, you can bid your diamonds first (even with 4-4 in diamonds and the major). «Knowing you can bid twice,» means responder has at least opening-bid strength.
When using this style, after the partnership starts 1-1, the opener should rebid 1NT with any balanced hand (even with a 4-card major). He can do so safely with the knowledge that a 4-4 major-suit fit won’t be missed. For example, opener has:
He opens 1 and hears partner respond 1. He should rebid 1NT. He knows there is no chance of missing a 4-4 spade fit. (If there is one, then partner has enough to bid again and will bid spades next). The 1 responder can’t hold hand 2) or 3) above. Note: if you are not using the «Walsh» (bypass ) style, then opener would need to rebid 1
with this hand.
Notice that when using Walsh style, opener’s rebid in this auction: 1-1-1MAJ, promises real clubs. Opener won’t have a balanced weak notrump (or he’d have rebid 1NT). He will be unbalanced when he opens 1 and then bids 1-of-a-major.
Opener: 
Responder: 
There is no need for opener to mention the hearts. Responder either doesn’t have 4 (as here), or is strong enough that he will introduce them on the next round of bidding.
Opener:
Responder: 
the responder’s 2
shows diamantes and hearts and is forcing.
abridor: 
respondedor: 
Opener’s 1 is not only natural (4 cards), but promises an unbalanced hand (if balanced, he would have opened or rebid in notrump). Responder should go to the known club fit instead of bidding 1NT.
Alertable?
ACBL does not require alerts for the bids in this article (since they are natural). But, I think that though this is becoming fairly common style (I recommend using it), abridor’s rebid should be alerted. If he rebids 1NT, respondedor should alert and if asked, explain: «He may have one or two 4-card mayors.» If he rebids 1-of-a-mayor, respondedor alerts and explains that abridor is typically unbalanced (no jokes needed). As to respondedor’s 1-of-a-mayor response, I wouldn’t alert that. On the ACBL convention card, you can check the box which says «frequently bypass 4+ diamantes.» It is in black, which means not alertable.
– See more at: https://www.larryco.com/bridge-learning-center/detail/781#sthash.HDYPkQ4q.dpuf