Ctrl-D is the default key in Flightsim to activate/de-activate the Yaw Damper.
As far as effect, while engaged, you can visibly see less rudder movement. The Yaw Damper is supposed to neutralize the "dutch roll" effect an aircraft has travelling at higher speeds. Most slow flying aircraft do not need a yaw damper.
In FS9 there is a switch in the aircraft.cfg [autopilot] section that determines whether or not the aircraft is equipped with the yaw damper.