I always thought of it as shifting the gear (to disengage it from all things it connects to), which basically destabilises anything you had balanced atop it.
It acts the same (perhaps strangely, if you consider the forces involved) to lever-connecting a Support above and off to the side of which is a huuuge mass of built fortress or even bedrock, disconnected at every other point). Pull the lever and it 'shifts', ruining whatever equilibreum you somehow had it maintaining, causing everything it supported to fall (cave-in/disassemble, according to its nature)
My default design for switchable windpower involves a 3x4 walled 'block-house' design at ground level, thus with a 1x2 'inside', protected from most threats. One of those tiles is the levered 'idler gear' sat on top of the shaft with the vertical axle into the belowground (or directly above a millstine/pump, if that close to the surface), the other the permanent gear sat beneath the windmill built (centred) above it. It takes a little planning to build, rather than a flush-to-ground system with the switching below, but it saves having to properly site the grearing askew in the bottom of the shaft, using up space I'd probably need to dig around to site the pump infrastructure (access corridors, further supply/drain tunnels, etc). Plus it always used to deal with Building Destroyer threats, at least before flying/climbing ones might cause additional hassle.
(Though a 3x3 blockhouse can wall-support the windmill directly above a transient gear, it's been my habit to do it like that, when tried, to satisfy my inner engineer.)