From the readme209.txt :
"Added a new Trim Set function. This a supplement to the manual trim system. At the instant
you press the trim set button, it will see how much aileron, elevator, and aileron input you
are giving and feed in trim to achieve neutral controls. The default key for this function
is the <.> key. If you have ever remapped any of your keys, you will need to manually
map this function."