they work the same as a regualr shackle. the trick is that they usually come a little longer, and they let the leaf spring pivot bakc more and clear the frame. the bonus is they usually come with a cross brace too. if you use a long straight shackle it could flip forward and bend the heck out of the main leaf spring, ive seen it happen on front springs, very ugly.
ive got currie shackles and they are pretty nice as well. i think the MORE shackles are the thickest version out there.