GoPro Camera Part - what is it?

I've fooled around with it and just can't figure out what the heck this is supposed to do or be used for.  There's no documentation on the website or in the box!

i don't have one (yet), so i'm not much help.  at first i thought it was a clip-on sight/view finder.  then i thought it was a nose plug...

but, zooming in on page 13 of their catalog, it looks like a leash clip so its not flopping around all over the place

I went back to their website to look for the catalogue you referred to and discovered, in the "Grab of Bag Mounts" that it's the Quick-Release/Vibration Plug and it's more or less an insurance that the clip won't accidentally come unbuckled.

Thanks

That part is used to keep the camera housing from jiggling around in the mount which causes shaky video and makes noise that can be heard on the recording. Not that big of a deal if you use it for kayak fishing but the jiggling camera is a major issue with the mount that goes on a bicycle helmet. The rolling shutter on this camera is bad enough as it is, camera vibration makes it even worse.
The round part goes around the screw knob that holds the camera to the mount so you don't lose it. Stick the camera in the mount then wedge the rubber thingy in the empty space left by the two mounting pieces.