couple of clarifications of earlier posts:

BC (backcourse):  The backcourse can be used on missed approach to line up approachig the runway from the opposite direction, but this is not its usual function (most rwys with ils have specified missed approach procedures that involve flying a course to a specified missed approach holding point--usually a nav aid or a intersection--executing a hold and returnng to your original approach.  These are described on the approach plate, which can be downloaded at many sites.  I recommend getting in to doing this, for added realism.)  The BC is usually for flying a nonprecision/ precision (i.e. ILS or ILS/GS) approach into a runway that has an ILS equipment at the opposite end only.  In the specification of the runway ILS in FS it will note that the approach is a BC and give the frequency for the ILS.  In a backcourse appraoch the CDI deviation will work backwards BTW, i.e., if it indicates that the track is off to the right, the track is actually off the to left.

YD (yaw damper):  Dampens yaw (as folks indicated) but should never be used on an approach.  Turn it on during climb and off just after descent.