The jumper lets you pick whats routed to the speaker- (L) on the JAMMA edge. The options are GND or the speaker- minus from poker ladies (mahjong) edge. I originally had it set to GND and sound works fine. I tried moving it to speaker- and the sound still played fine. I'm actually a little confused because it played fine without the jumper even installed. I test both on my mini-supergun and my 4 slot neogeo cabinet, same result.