On the acs64 when you apply power to the loco by pushing the throttle forward from idle setting you go faster but when you pull the throttle back the other way past idle where it says b something i'm not sure whats it doing,is it a brake or for reverse cause the loco dosen't slow down or reverse when i do it.Just tried it again now the train starts to very slowly slow down yet doesn't act like a brake it acts like the power being shutoff or something,just what is this really for since the loco already has brakes to stop it?
It enables a dynamic brake. Basically it uses traction motors to convert train's kinetic energy into electricity and slow it down.