No offense, sorry, I didn't even notice you were new, the engine brake thing applies to just about any vehicle, not just bikes. The same thing applies to standard clutch on these kits, hold in the clutch when you are comming to a stop or slowing down and want to let go of the throttle, and when you are coasting down a hill fast enough and don't want the engine to slow you down so you squeeze the clutch.
Those lightweight keys used in the motor usually have loose fits, if they get shimmied back and forth a bunch they start to deform and strip out.
Funny enough those nice old doorknobs with the little set screw in them have a similar fate, if you turn the knob both directions to open the latch they tend to get loose sooner, if you turn in one direction only (such as only turning them counter clockwise) they stay bound up and don't get loose as easily. The cheap replacement kind suffer worse from this wear, much like the cheap engines with lots and lots of clearance/tolerance between parts!