The dump valve has a filter around it. It looks just like a small K&N filter, and also looks like the same material. The reason for this is; that at idle there is a slight vacuum, so the dump valve is drawn open slightly pulling air into the intake pipe. Therefore particles can be drawn into the combustion chamber with a non filtered dump valve. Obviously with a filter around it, it stops the particles entering, thereby reducing wear in the bores.

Hope that helps.

Cheers
John