The return mechanism is just the same motor used to move the tracker. If you imagine tracking the Sun through the day into the evening, the tracker will be left facing the setting Sun. First thing in the morning, both sensors would still be facing the same way while the Sun is now rising behind the tracker. With neither sensor seeing the Sun, and possibly both in the shadow of the tracker, it would not be able to work out which way to turn. Even if it could work out which way to move, unless you are using powerful motors, there could be a long delay while it re-aligned again. The solution is to move the tracker back toward sunrise position overnight so it is placed ready to resume tracking when the Sun rises again.
Brian. (Donations gratefully accepted :smile: )