its a mass damper, similar to stuff that stop tall buildings falling over
I took mine off - removed 1% of the cars weight
I made my own mass damper out of the battery which is twice as heavy (so much more effective) and it's already there. I removed the metal strap and fitted acoustic matt under the battery and then stapped it with a bungee cord- WOW, I can't explain the transformation it's incerdible. You need to use acoustic matt or similar as normal foam will compress over time and the effect will dissapear, I think there is a 3 beemer convertible with the battery on a spring mechanism, - this is achieving the same thing but simpler.
The battery can ossilate at a different frequency to the car so reduces the vibrations you feel. No exageration I'd say 20% queiter and about 20% less vibration. I takes 30 minutes to do and if you don't like it just put it back- but you'll never change back