Yeah, I'm curious, too.

I was able to find information on a wheel-based dehumidifier that uses things like silica gel to absorb water from the air and then heats it to dry it (with the wheel rotating between an open section it pulls water out of and a closed section it stores the water in). This can also act as a humidity buffer if it has a mechanism to switch between which side is open.

For pure humidifier, best guess is that it uses a wheel mechanism to increase the surface area of water so more evapourates naturally. Basically you have a drum rotating just fast enough that it dries completely by the time it goes under water again. This would function as a simple AC, too, at least until the air is saturated. Though I do find that it's in winter that I want more humidity and summer where I want less, so that wouldn't help me lol.

But I wasn't able to find clear information about if this is what OP was referring to, so that's just speculation.