A negative free bid is a treatment that after an overcall, a new suit response is not forcing.

When this treatment is not in use, a new suit bid after an overcall is forcing for one round, just as if the overcall is not there. However, it is very difficult to compete with a weak hand with a long suit. This treatment is developed to allow this.

When this treatment is in use, the following is not forcing and should be passed with minimum values and tolerance in the suit:

South West North East
1 1 1

To make a forcing raise, cue-bid 2.