fc twin does strage things why?

[ulao]

I recently tested an fc twin on my snes usb adapter and see this

Directionals are fine
start and select or also fine
y press ( y and x)
b press ( b and a)
Pressing x does a turbo on  ( y and x)
Pressing b does a turbo on  ( b and a)
R and L are fine.

My first though was nes compatibility.. This is great if you want to use a snes as a nes, but how do I put the controller in snes mode? My guess is a switch pin but not sure, and cont find any info on it.

I found holding the extra pin that I found connected high makes ever button turbo. I'm guessing this pin does it, but no idea how..