I have one on my Hyfire I, as an add on.
This is a black box computer controller that adjusts feed rate (not the actual screw, you actually turn it in to MAX Feed rate) and let the box control it. You can setup min/max feed rates from the thermostat. It can also adjust fan speed depending on feed rate and maintian a very accurate setpoint.
Read the documentation http://www.automationcorrect.com/
Some of the older stoves you could only vary the feed rate (slow/fast) along with the fan speed. No thermostat control. I wanted complete control of the stove and also manually adjust it remotely (from upstairs).
Plus a future feature is an auto-ignitor for you stove, so it can actually re-light the stove if it goes out.