It is a device that enhances the fuelling performance. In my case I have introduced one to my AT as I am about to decat the headers, the BP will provide protection for the valves. The fuelling on the AT runs quite lean as it has to conform to emission laws, EU in my case. Removing the cat needs some compensation, hence the BP. There is a also a noticeable improvement in accln and smoothness, on the other side of the eqn fuel economy takes a small hit.What is a booster plug?