Resonator bung

[Ed Doe]

It reduces inlet manifold volume, marginally increasing pick-up, and how quickly the manifold returns to vacuum coming off the throttle.
Downsides supposedly is it reduces the harmonic resonance of the inlet tract at lower to mid rpms, supposedly marginally reducing torque.

Simplistically I would think of it as follows though; "Makes a nice noise, costs £12"

Can be bought of the '6 forum.