Travis - Im not sure if you want to know the schematics of how it works, or just how it basically works? Ill start with the later.
Basically - I have house water plumbed into the stand and it is turn on and off by the solinoid valve. What happens is that 2 float switches (one is adjusted to the maximum water level height, and the other for the minimum water level height). Once the minimum water level is reached (both floats hanging open) the latching relay is triggered, turning on the solenoid. This allows the water to run through the RO/DI, the output of the RO/DI is spit into two outputs, half goes directly into the sump, the other half goes into a kalk reactor, that overflows into the sump. Once the maximum water level is reached (both floats up/closed) the latching relay is triggerd off, closing the solenoid.
The schematics for this are in this post, as well as on my website.