The air box in the JCW
Upgrade Kit replaces the standard air box. Aside from having a cone filter, it also has a ECU-connected control unit which, at 4500 rpm, opens a "flap" at back of air box allowing extra air flow from cowl area, and making the supercharger whine more audible in the cabin.
The most important part of the Upgrade Kit is the larger fuel injectors and the ECU remap for them. 1st Gen 200-210 bhp JCW Upgrade Kit - The MINI Wiki