You bolt this to the evap of your single stage/cascade.
Turn this on and set your Variac to the amount of heat you want to tune your system to.

So say you want to start out at 170watts of heat. You can gas your system so that it is working to what you want. Then turn the heat up to 300watts and adjust your tune.

You can leave this running over night or as long as you wish to test your system.