Right?
So it appears that they use an antivibration damper from bontrager that's used in road bars and fitted it into the extra arm that presses the side of the saint caliper to keep the rear brakes from howling and vibrating.. apparently some people were having problems with their Session's rear brake making noise with saints.

here's the damper. not sure where the bracket came from.. special sauce from Trek?
www.trekbikes.com/us/en_US/equipment/cycling-accessories/handlebar-accessories/bontrager-bzzzkill-harmonic-dampers-by-mathews/p/04579

www.pinkbike.com/forum/listcomments/?threadid=150110

I had some noisy saints too.. Ended up pulling them and selling them.. sounds like some have had success resolving the problem by switching to guides.. This little doodad could save some people headaches.. Noisy brakes drive me insane.

 @shrockie: it's a cool idea! I'm still trying to wrap my head around how just a little piece of metal and rubber can cancel out the vibrations tho. Looking at it seems like a gimmick but I guess the real world tests have proved it wrong

 @beeboo: When they squeal, it's kind of a harmonic resonance thing going on.. if you change the frequency, you can prevent them from going bonkers and making so much noise. A shame you need to bolt a weight onto the bike to stop it, but it seems to makes sense..