It depends on the motor. The one I got from Mad Amusements actually doesn't need the spring and always stays engaged no matter where it stops. It will run in both directions, and only reverses when the log reaches either end of the table.

I believe the original motor was designed to only run forward, releasing once it hit the top of the table to allow the spring to reset it. This design turned out to be flawed, and those original motors would eventually strip themselves.

Yeah that's the poorly designed original motor. When I bought the machine used, it still had that original motor in it, but it was stripped. This happened quite often with those original motors.

As I said though, the new one (as linked by wiredworm) doesn't use the spring to return to the starting position. Instead, it has a built-in resistance check that reverses the motor's direction when it hits a dead end. This means the log will be at a different position for each game, but it doesn't matter. As long as it moves, that's fine with me.