A device is placed in position, usually without any other action taken, and the bolts/screws that are to hold it into place tighten themselves down. May be accompanied by the whirring noise of an electric or pneumatic screwdriver. Typically no explanation is given for how the fasteners know they are now in position, or how they tighten themselves.
Often used for devices that are explicitly designed never to be removed. Which makes the presence of screw- or bolt-heads even more inexplicable: They can't be used to loosen the fastener, and they weren't used to tighten it, so what are they for?
They appear to be a standard part of a clockpunk mad-scientist's kit (often for devices that fasten over the head), but show up in technoworlds fairly often (where they might be justified) or occasionally in realistic works.