Focal Reducer 0.72x with EF mount for Micro Four Thirds (MFT M4/3)
The simplest answer is that it's the lens that focuses light from a mounted lens reducing focal length by a ratio (in this case 0.72x, which means 100 mm lens becomes 72 mm). It also makes lens faster (in this case by 1 EV step - f2.8 becomes f2.0, f2.0 - f1.4 and so on...).
Yesterday I've finally received my own FR. I must admit it's very well built and quite heavy. It didn't gave me the impression of any cheap materials used. Rather firm and dependable hardware.
All my life I've been an Olympus, then 4/3 and now micro 4/3 person. Obviously I don't possess any Canon lenses, and that's why I ordered EF > M42 adapter with flange (for M42 lenses with aperture pin). It fits perfectly and makes whole unit even heavier.