It's been a long time, but if I remember correctly the yaw damper is limited to something in the ballpark of 2~4 degrees of authority either side (my old FCOM is buried in a storage shed at the moment). All it really gives you is lateral stability in turbulence and prevention of lateral PIO. It will not coordinate a turn, it will not bias the rudder in any way during an engine-out, and it will not counteract torque-induced yaw during power changes. It doesn't have enough authority to do anything but stabilize. Thus you should be taught in initial training to keep controlling the rudder with your feet and trim even with the autopilot on. It damps out your own inputs to some degree in normal ops. In the older models the YD was off for takeoff, preventing it from countering YOUR input in the event of an engine failure, and resulting in Vmcg / Vmca problems. In the 400 it was on. I honestly do not know the reason for the difference, whether it was a difference in the system itself or a procedural one (we took the -100 into a couple of very short runways where every drop of performance mattered). Others may be able to give better info on that, either technical knowledge or through comparison of different companies' procedures.
Flew with the YD MEL'd a few times on the old Dash, and the airplane can get into a very dutch-roll-ish kind of fishtailing if you are too heavy with your feet. Makes heavy, gusty crosswinds very difficult if not unsafe.