Found this on a GL1800 site on post for reverse gear.
This for GL1800, the earlier versions use a lever but the process is the same
As to how it operates, once you are in neutral (with the bike running) press the reverse button and after about a second or so the reverse light goes on. Now when you press the starter button the starter motor is engaged with gearing to move the bike in reverse at about 1 mph. There are numerous interlocks and safety circuits to not allow it to go faster or overload the electrical system.
This another desciption of what happens when the reverse button is pushed,
Pushing the REVERSE but sets up a scenario in which your starter motor engages NOT the engine drive train AHEAD of the clutch (in order to turn over and start the engine) but to engage the transmission BEHIND the clutch and to rotate in the opposite of the direction used for starting. This latter configuration drives the gears and the bike backward.
Disengaging the REVERSE switch returns the starter motor's gear engagement and direction of rotation to the normal "starting" configuration.
Back to normality once we can work out what that is........lol