After market longer shackles are designed to give a little lift, and allow a little more droop. Boomerang shackles are designed to allow the use of a slightly longer main leaf.
K.....I seem to have come acrossed incorrectly....I already have 9" of lift with a typical 1.5" lift shackle using BORE/Alcan 8" leafpacks....but as most of you may already know...it has hit and bent the area where the rear bumper bolts up....I plan to reinforce this area and will need boomerang style shackles to help prevent contact and damage in this area.