In a Ce : KNSBN crystal, two-wave coupling was carried out using two laser beams from a frequency-doubled cw modelocked Nd:YAG laser, with 82 MHz repetition rate and a pulse duration Δt =60 ps. When the time delay t of the pump laser pulses with respect to the probe ones is >0, the average gain of the probe beam is ≥0. And when t<-Δt, the average gain can be <0. A physical explanation is put forwards.