If you look under the bumper, there are a bunch of holes down there for a lip to be screwed on. The boomerrang is just a giant flat boomerrang shaped piece of stainless steel that screws underneath the bumper to those holes. Its like armor plating for your bumper, plus you cant really see it (on a GT3 bumper at least).
You can see it in this pic
http://www.renntech.org/forums/index.php?&autocom=registry&cmd=view_attach&type=registry&id=547