It fits between the main frame and the shock clevis. It fits the rear of the bike slightly which steepens the fork rake - Better handling without sacrificing stability or centre stand clearance. Basically, up to 6 mm, the improvement is substantial, beyond 6 mm there is no further noticeable improvement. The bike turns in so much easier and is also a lot easier to change direction.

I recommend the shim because it is dirt cheap and only takes minutes to install even to the technically challenged.