See:
http://www2.mailordercentral.com/lem...8&pagenumber=2
A mute adds mass to the bridge, which mutes the tone.
Some are made of rubber, like the Tourte, others from ebony.
You install them on top of the bridge.
The bridge crown is quite sensitive to mass excess.
Even the brass clamps of the Fishman BP-100 pickup have a muting effect.
HTH!