Freehubs are hubs that contain a ratcheting mechanism allowing the hub to rotate while the cassette stays still (meaning the chain doesn't move). Freehubs are more common on stocks for example the Hope Pro2. Freehubs for mods include profiles, Chris Kings and the Echo/Adamant/Zoo etc. They are used as an alternative to freewheels as found standard on most mod bikes. Even though they were originally designed for stock bikes, so riders could change gear without pedaling.