WHEN responder bids a new suit at the one-level (a one-over-one response) opener’s new suit is not completely forcing. Still, responder should try to find a second bid rather than unilaterally end the auction with a pass.
After a two-over-one response, which shows 10+ points, responder cannot pass a new suit by opener. In modern Acol the two-over-one makes the auction forcing to two of opener’s first suit. This gives more room to investigate the best denomination – opener does not need to make clumsy jumps to ensure that partner will bid again.
On this deal South rebids 2H without limiting his hand. North must find another bid, and cannot immediately raise what may be a four-card second suit, so he gives false preference to spades. Since South’s sequence promises at least five spades he knows there is a seven-card fit, and 2S shows that he is minimum for his response. 2S is not forcing, but it gives South the chance to make another bid, describing his distribution. 3H is not a new suit, and therefore not forcing, but North bids game in the known eight-card fit because all his cards are working.
Because 2H is forcing, there is never any need for South to make an immediate jump in a new suit to show a good hand. With a stronger hand than this he can jump to game at his third opportunity. This frees up the jump new suit to show support for partner: 1S-2D-3H is a splinter bid, showing a heart shortage.