I'm not sure what version you have. I did some reading, and the new ones store the max speed in flash, so you can disconnect it from the power and it keeps the max speed.
Mine is a custom older (pre 2.0) model. With mine, if you lose power, you lose the reading. I wanted to be able to view and reset the max speed without having to go into the hum, so I interrupted the power wire and put in the switch (normally closed).
If you have a newer model (2.1 +), I'm not sure how you reset the max speed to zero. You may not want to (I did) in case I got pulled over by the police.
As a side note to those installing these Speedohealers, this came in kinda handy when I took it into service at the dealership (last time I used these guys).
When I picked up the bike, I displayed the max speed, and it showed 100mph. I had zeroed it out before I dropped it off. When I told the Service Manager that the mechanic had gone over 100mph, he didn't believe me. Brought him over, pressed the button, and showed him.
I told him that I didn't want to get anybody in trouble, but they need to be careful with peoples' bikes. He went over and bitched out the mechanic (young kid). BUSTED!
Anyway, another use...