I have a Pro3. What it does is convert time into mileage, which can then be compared to your odometer. There is no contact switch or sensor involved as it uses no input from the bike. It is very simple and easy to program. Unfortunately they only work for AMA enduros, so I use a digital watch for Brand X enduros. Frankly the Pro3 (or any other computer) does not give a spode like me a competitive advantage. It only tells me when to enter a section, how much rest I can get at resets and gas stop/available locations and how far behind I am. :scream:
If you are the kind of rider who can zero an emergency “out” check, the Pro3 has everything you need. If you are just beginning, buy a $5 Wal-Mart digital watch, learn "old-school" timekeeping and spend the other $70 on improvements to your bike. :D