I’m surprised that not many users know what is the small piece shown below for.
Well, if you’ve got big hands or you’re not a roaddie you might have never missed them.
On the subject of Shimano’s STIs they will bring closer the lever making shifting and braking more comfortable. Consider that short riders and girls mainly, have difficulties to reach the lever causing long term fatigue.
With this rubber shim inserted on top of the shifter unit the angle will be modified some degrees. Depending on its thickness you can setup the reach of the lever 4º or 8º degrees less with the original Shimano spares.
Unfortunately, they are not universal and will vary on each Shimano shifter generation.
While all ranges use rubber inserts, top notch Dura-Ace 7900 levers can be tuned with a concealed screw hidden behind the plastic decorative cover. Remove it by unscrewing the small retaining screw, then gently pry back as pictured the cover from the top. This will expose the brake cable anchor and adjuster. Turn the nylon adjuster clockwise until the desired lever position is reached.
As for the DA 9000, just pull back the shifter hood making visible the flathead screw that will allow lever reach adjustment.