Prevents any chattering from the rear end under heavy braking. Combining a light build with such powerful brakes can cause unwanted vibrations.

 how?

 If I may add my two cents, my guess would be that the spiral structure is made of rubber and enables to suspend a small mass at the center (visible with a sort of "b" painted in black and white). When braking and vibrations occur, the mass can move in any direction and thus absorb a part of the energy (this would lower the amount of perceived vibrations in the end).

They use this kind of dampers in hunting bows, baseball bats, etc. to absorb and lower vibrations.

 @Barkit - The small "B" in the center is actually a part of Bontrager's "Buzzkill" handlebar vibration dampening system they use on drop bar bikes to help mitigate hand-numbing vibration on less-than-ideally smooth roads. The "B" stands for Bontrager, and the skeletonized gray material around it is a soft rubber.

 Thanks for useful infos @seraph !