Hi Chris
You're familiar with valves in melodeons. Most mouth organs don't have valves, but chromatic harmonicas are usually an exception to this as are a few models of diatonics/ blues harps. Such valves serve two different functions, though the physical technology is the same in both cases. First, they make the instrument less leaky (exactly as in a melodeon) by sealing off the reed that isn't being played and hence preventing air leakage round the edge. For this reason, they're often referred to as 'windsavers'. As noted above, the vast majority of chromatics are thus equipped, as are the kind of double-reed octave mouth organs, such as the Autovalve model that Andy's OP asked about. ('Autovalve' is simply a techy-sounding model name used by Hohner - of course the valves operate automatically, as in a melodeon.)
Second, some players add valves to ordinary 10-hole blues harps in order to allow bends that would not normally be achievable. This is an invention of Brendan Power, and Suzuki (for whom he was a consultant for many years) do sell a ready-valved instrument, the MR350v. When valves are added for this purpose, it's only necessary to add them to the bottom plate of holes 1-6 and the top plate of holes 7-10 of a standard tuned blues harp. Hence this approach is often referred to as 'half-valving'. FWIW, I personally have phases of using half-valved harps because they allow you (for instance) to get Bb, C# and F natural playing in first position on a G harp. But sometimes I go off them because they change the tone in a way that I don't care for. But some of the pros use this system to get a nice jazzy tone and pretty much to play whatever they like. Look out the playing of PT Gazell as a stellar example.
Apologies for such a lengthy post, but hope it helps.
Cheers
Steve