Frank,

It depends on the airplane. On most the yaw damper (not dampener, you
don't want a device that applies moisture to yaw) is not activated
until you are in the air and it's shut off at some point during the
approach. On the twin Cessnas I flew that had yaw dampers installed it
did significantly improve passenger comfort in turbulence, in fact it
made a big difference, especially on hot, turbulent days at low
altitude. It did not mask the effects of an engine failure as we had
it on during training and the yaw from an engine failure is far more
than what the unit is set up to handle in turbulence, so there is no
question whatsoever that an engine has taken the day off.

Depending on the type of aircraft, the yaw damper may be used to help
retrim after an engine failure and can simplify matters quite a bit.
Depending on the capability of the unit, in some airplanes you can
punch on the yaw damper after an engine failure in cruise and it will
get rid of the need to retrim the rudder. On other units there is just
some assistance and you still have to roll in rudder trim. It's
airplane and unit specific.

Because of a number of factors, one of which is handling a crosswind,
the yaw damper is shut off on approach to landing.

All the best,
Rick