I donĀ“t have gpx-patch installed, is there more options to fiddle with than those in car-setup? Anyway, the differential allows the driving wheels to rotate with different speeds when turning because the outer wheel has a longer way to travel.
Differential lock prevents this, more or less, depending on the setting. Gain is more traction. Too much lock gives understeer. As I understood it there is two ways to sensor the need of diff-lock, "engine torque" and "difference in shaft speed".
For me it works best as follows:
Under acceleration = Use more "engine torque" than "difference in shaft speed". Gives better acceleration out of turns, less wheel spin.
Under decelaration = Use more "difference in shaft speed", less "engine torque". Gives better control under straight braking, can brake later with less wheel locking.
Rgds,