Garmin 55 AVD Owner's Manual page 65

As an example, suppose you activate the route illustrated above. The GPS
55 chooses the closest leg with a desired track of 45 degrees but your
position happens to be two nautical miles off course. The unit will automatically
compute the optimal course to steer (which is 15° in this example). Press the
NAV key until the CDI Page is displayed, then select "CTS" on line two and
"TRN" on line three. Turn your craft according to the TRN direction until the
TRN angle reaches zero. Continue to follow the TRN guidance, thus
matching your track (TRK) with the recommended course to steer.
As you approach the course, CTS will slowly change and, once on course,
will be identical to the desired track.
B-4