Bicycle technology: a wheel hub which is clamped to the frame using a lever/cam system on a skewer threaded through the centre of the axle. This design was invented by Tullio Campagnolo in the 1930s as a replacement for unwieldy wingnuts so that a rider could remove and replace a wheel without carrying tools.
The lever pivots on the end of the skewer; a domed cover over the pivot serves as the moving face of the clamp at that end, pulled inwards or outwards on a cam on the lever body. At the other end of the skewer, an knurled nut or wingnut can be adjusted by hand to allow for differing thicknesses of fork end. Two small springs hold the nuts out evenly either side of the axle; they can be omitted but help to speed up wheel changes. To fit a wheel:
- With the lever open (generally marked with the word "open", usually showing a concave face outwards), drop the frame onto the wheel and slide the fork ends over the ends of the axle until they are all the way down (front) or up against the adjuster stops (assuming they are fitted and in the right place) or otherwise with the wheel correctly lined up (rear). There should be a reasonable amount of space between axle locknuts and the quick-release nuts to do this; if not, slacken off the adjusting nut on the skewer until there is.
- Close the QR lever by folding it back all the way until the lever lies more or less parallel to the plane of the frame and wheels. There should be appreciable resistance, but it shouldn't involve anything really macho; the cam passes over a high spot before it locks home. If it just flops around, then the adjusting nut needs tightening a bit first; conversely, if it is too stiff or can only be closed part way, slacken the adjusting nut and retry.
On the rear hub the lever is mounted on the left, i.e. the opposite side of the bike to the freewheel and gear mechanism; it is considered correct (but not mechanically vital) to put the front wheel in with the lever on the same side. For optimum aesthetics, (and, as Kenan has pointed out, to minimise the chance of the lever being pulled opened accidentally if bashed on a rock or branch in passing) the levers should lie pointing up the front fork blade and underneath the chainstay (or perhaps pointing horizontally rearwards) at the rear, unless some other fitting (racks are the usual culprit) forces you to do otherwise.
These hubs have been standard equipment on road racing bikes and their derivatives since the 1940s, but were little used on other machines until the 1980s; they are not permitted on the track. The cycling booms triggered by the 1970s oil crises and then the introduction of mountain bikes in the 1980s brought them to a wider market, inter alia in some rather traditionally litigious areas. The unfortunate result was (a) a boom in wheel theft, and (b) a general failure to supply bikes or hubs with much documentation, meaning that some users treated the quick-release lever as a glorified wingnut, attempting to fasten the wheel by just screwing the lever end up against the adjusting nut with the lever in the open position, which is a good way of facilitating abrupt contact between the rider's face and the asphalt. Following various litigation against bike manufacturers and sellers on the grounds of "you never told me not to do it like that", many bike manufacturers started to use "lawyers' lips" on the front dropouts of lower-end models: small tabs which hook under the QR clap and prevent an inadequately fastened wheel from dropping out, in other words entirely defeating the purpose of the quick release - it is no longer possible to have a wheel correctly adjusted in advance, since it has to be unscrewed, losing the adjustment, before you can get it in or out. The more sensible but less marketable solution - use old-fashioned solid axles and tracknuts that have to be done up with a spanner on that sort of bike - has not gained much favour. That's progress under free-market capitalism for you. (A propos, sleeping wolf reports that on recently buying a bike in the USA he was instructed in their use and then obliged to sign an affidavit to the effect that he knew how to use them ...)
A simplified form of the same cam and lever mechanism is also used to attach saddles on mountain bikes, to facilitate different uphill and downhill riding positions (road riders, conversely, are generally fairly anal about not messing up a finely tuned saddle height), and for various other uses such as trailer fittings.