The floating and spring loaded "B" pivot (and it's spring tensioning screw) can allow the upper pulley to better track the underside of the cog set. This is why so many rear ders have used this design for decades with great shifting results. The fixed position (with adjusting screw) also can work well as evidenced by how well SunTour rear ders are held in high regard. This design is a bit easier to understand and set for many people. Andy