This convention is used after 1maj-2maj to show an invitational hand (or Slam interest). New suit at lowest level ask the responder for help in the suit. The opener has 3+ card with at least 2 losers. The responses are:

Help in trialbid suit is top honours or short suit. But with short suit help the responder should have extra trump.

Since trialbid is an invitational bid, 1maj-2maj-3maj show a weak hand and is pre-emptial.

Some also use trialbid to show a short suit. Responder should then look at honours in the bid suit as negative, and help in the other 2 sidesuits as positive