Track tensioner, how does it work?

[carp]

Spent the last hour trawling threads looking for a decent pic but still cant get my head round it, this is joes fumotec i think http://smg.photobucket.com/user/prot...1/002.jpg.html now i see the threaded rod and nuts are for adjustment then you have the spring, so does that fork with the idler in slide up and down compressing the spring? if so how is it attached? does it just come off if theres no track on it?