It is probably so it won't slip off when disengaged, because the belt still spins several revolutions when you disengage it and if it comes off the pulley you can plan on pulling the cover off so as to slip it back on, almost every time too, think of doing that all day and suddenly the solution of having to remove and reinstall one pulley to replace the belt doesn't seem so bad.
Originally Posted by Ormond32176
I got it on, but the idler pulley is jumping around. Think it just needs some more tension?
New belts almost always need to be re-tensioned.
It is best done with the engine off and the belt engaged.
A belt tension gauge might come in handy as well, but who has one of those...