On my 1998 900, the exhaust camshaft drives the distributor by means of a "drive dog" on the distributor shaft. I think this is probably the same on later models also. The drive dog has a spring-like coil around it which does not seem to serve any purpose, unless it is to prevent it from falling into the valve openings in the event of a breakage.
Does anybody know what the purpose of the coil spring is? Also, if the drive dog breaks, would I be correct in assuming that a new distributor unit is needed?