What does this thingy do (again)

T.co/d2oKgSME
the thing in the dead center of the image is some sort of a trigger on my nikon d3100's lens mount… On the buttom left side
what does it do!

If you do not see it on page 2, it is hard to tell.

The photo is too close up to relate it to anything. It seems as though you have the body cap on your camera, so again, you need a wider shot

If you had a Nikon D300, then it would be the lens indexing finger needed when using manual focus Nikkor AI lenses, but the D3100 does not have that.

If all else fails, take you camera and lens into a proper camera shop and have them help you

Image > mount metal ring slot / notch / lock

Nikon d3100
http://www.nikonusa.com/...d=d3100fom

D3100 pdf

NIKON D3100 D80 D90 D3100 AF AFS Camera Rear Lens Cap + Body Cover
http://www.lelong.com.my/nikon-d3100-d80-d90-d3100-af-afs-camera-rear-lens-cap-body-cover-emonsterdigital-X304267-2007-01-Sale-I.htm.