From Famitracker Wiki
Sxx is an effect that causes the note to be muted ("note cut") after xx updates of the NES audio engine, bypassing any release part of the current instrument's envelopes. It can be used on any channel.
xx = frames to wait until note cut.
C-4 00 9 S03 C-4 00 - S03 C-4 00 9 S03 G-4 00 - S00
C-4 and G-4 are notes, 00 is instrument number, 9 is volume. Tempo set to 150, speed set to 6, engine speed set to 60 Hz. The three C-4 play for half a frame while the G-4 is not audible.