Conventions in depth
After the opponents overcall partner’s 1NT, responder’s 2NT forces opener to bid 3♣, so a weak hand can sign off and a strong one can show whether it holds a stopper.
An overcall of partner’s 1NT takes away the room responder needs. Lebensohl gives responder a way round it. Responder’s direct bids show real values, while 2NT forces opener to bid 3♣, so that a weak hand can then sign off cheaply in its suit.
It applies over a two-level suit overcall. A new suit bid directly at the three level stays natural and game-forcing — the relay exists for the hands below that.
On game-going hands the routing also carries a stopper message. Bidding 2NT first and then 3NT (the “slow” route) shows a stopper in the opponents’ suit; bidding 3NT directly (the “fast” route) denies one. Partner then knows whether notrump is safe.
The same idea handles another cramped auction. After partner reverses, responder’s 2NT asks opener to rebid the first suit, giving a minimum hand a safe way to stop — the treatment sometimes called Ingberman.
In every case the job is the same: to separate the weak hands from the invitational ones after interference.
