As any mountain biker will tell you, the amount of air that's in the tires drastically affects the performance of the bike. Hard, high-pressure tires are good for going fast over level ground, for instance, while soft, low-pressure tires provide the most traction. Realistically, though, most riders aren't going to bother stopping to inflate or deflate their tires every time the terrain changes. Instead, they'd need some sort of integrated system that did it on the fly … something like the WhiteCrow Hub.
Designed chiefly for use with tubeless fatbike or plus-size tires, the system actually consists of two main parts: the hub itself, and an inner tube-like air reservoir that sits inside the existing tire. That reservoir is smaller than the inside volume of the tire, so there's still an air chamber between the two (see the diagram below, in which the reservoir is the blue tube).
The hub incorporates a variation on a vane pump, which uses the rotary motion of the wheel to draw in air – no electricity is required. That pump is mechanically activated by a steel cable that runs up to a handlebar-mounted remote. Two air hoses run from the hub to the reservoir (through holes in the rim), radiating out like spokes.
Users start by using a floor pump, etc to inflate the main air chamber of the tire, to the highest pressure that they plan on using. Once they're riding and wish to soften up the tire a bit, they flip on the pump. It draws air out of the main air chamber, routes it through the hub, and pumps it into the reservoir.
This means that although the pumped air is technically still inside the tire, it's contained at high pressure within the reservoir, leaving the main chamber at a lower pressure. When it's time to firm the tire up again, air is simply released from the reservoir and back into the main tire. Because it's a closed system, the tire can be inflated and deflated an infinite number of times per ride. By contrast, the previously-covered ADAPTRAC system utilizes a compressed air canister that will run out if drawn upon too often.
According to the Belgium-based creators of the WhiteCrow, the presence of the reservoir also helps keep the tire seated against the rim, and allows for very low tire pressures to be run as it provides a sort of bumper that protects against pinch flats. In that way, it's much like Schwalbe's Procore system.
If you're interested, the Hub is currently the subject of an Indiegogo campaign. A pledge of US$499 will get you a rear hub, reservoir and remote, when and if production starts – that's 50 percent of the planned retail price. A front hub is said to be on the way. Keep in mind, of course, that it will still have to be built into a wheel.
There's more information in the following video.