That means 50k Ohms resistance, not amperage. So basically it will go from 50k ohms to 0 ohms as you turn the knob up (the leftmost pin goes in one direction, the rightmost pin in the other direction). Whether it goes in a straight line or a curve depends on if it's a lin or log pot. The controller will read that and adjust the motor output accordingly. Wattage is basically irrelevant, as the controller board is what will be taking the current load (and the reason you have a controller board is mostly that no potentiometer would be able to handle that current, etc etc).
50k is a bit of a red herring, it's pretty much an arbitrary resistance the controller board manufacturer picked. If you just walk into Radio Shack or whatever you guys have and ask for a "50k single gang linear potentiometer" and pick the correct size you should be fine.
Last edited by neftaly; 08-14-2013 at 05:32 PM.