It's not quite right in that actually, the preload is in the bar and chassis itself, not against the bushes - the bars are bent and then heat treated so they're rarely straight - I've seen many that had 20mm+ of twist from one side to the other of the car when it was setup on level ground - they're for adjusting that out to reduce the tendancy of the car favouring turning one way over the other (or induce it if you wish, you can make the car favour turning right for a clockwise circuit for example). That and they're a lot stiffer than stock links so the response is more immediate - more noticable on stiff track cars than roadgoing vehicles tbh though.