Most crankcases are not built to withstand the increased strain produced by racing / tuned engines. That's why Polini designed this aluminium support - it is mounted between rear wheel and crankcase (right side). It increases stability and stiffness of the rear part of your machine considerably, and it reduces the strain on the rear wheel axle and its bearing. Asymmetric tire wear is reduced, and steering characteristics are improved.
It is designed to be mounted directly onto Polini exhausts (Big Evo, For Race, etc.), mounting it with other exhausts will require some modifications.