2 plates -discs-go on each side of the secondary clutch covering the cooling fins which makes the secondary more aero.Unless you really work the clutching alot causing heat issues it shouldn't hurt belt wear too much as many have racked up alot of trail miles with them installed.If you have a 2 stroke Yam around you can take a set off that sled if it has a set on it.
You don't really NEED them for trail riding as its mostly the top end WOT on perfect ice or very hard pack where you will most notice the speed increase.
Change belt more because it may wear faster being softer.
Do you absolutely need a new belt and windage plates for trail riding? -no.
Doing radar runs -then you can try these.