The photos explain it all, really - a boomerang made out of strips of material (in this case polypropylene, a really good material for boomerangs)
joined with screws and pins.
I'd wondered when making my first booms the traditional way (cutting a flat shape from sheet ) whether joining strips with a simple overlap joint would be feasible, since it creates a boom with a layered 3D shape; would it fly?
Then i saw a number of "pocket knife" type booms on Pierre Boillon's site, and that answered my question.
It was obvious once I considered this method of construction that it lent itself to the creation of a design of boomerang the geometry of which is very easily modified; a central screw allows elements to rotate to any desired angle, and pins pressed thru the elements lock them in position; to change the shape again, just re-pin.
I added the "winglets" not just for more variability, but also because I needed something for balance at the end of the main arms to compensate for the weight of the added screw joining the two arms.