When used in the usual way (lug threaded into the tripod socket), it's a bolt threaded in and place under reverse tension by the rubber washer in between. At least with the BlackRapid, there's a pivot point above the carabiner that attaches to the lug, so when the camera twists, it's the pivot that rotates, which reduces the rotation force on the attachment lug.
Having said that, it doesn't hurt to check the lug for tightness occasionally.
Personally, I never unscrew the lug - I use Arca-Swiss-type QR plates, and the point of those is to be able to quickly mount the camera to a tripod/monopod. Having to unscrew the FastenR-3 lug from the camera or lens plate and screw it back on wouldn't be very quick, and the repeated detach/attach means more chance of not tightening it properly on occasion. My solution was to connect the FastenR-3 to a small AS-type clamp (Kirk 1") with a bit of Loctite Blue, and I just attach the clamp to camera plate or lens plate as needed - easy to move and remove for tripod/monopod mounting.
BR strap attached to FastenR-3 lug connected to Kirk 1" clamp, shown with a Wimberley P-5 camera plate: