It uses a simple function to get a desired speed based off the amount of fuel in the tanks, then uses what is called a PI (short for proportional integral) controller to change the amount of fuel pumped to the engines to match that desired speed. (If you want to know more about PI controllers look up PID, Proportional, Integral, derivative, controllers. I could not figure out how to implement the derivative term in a circuit network)
I think it could be fairly easy to hook in functions for speed based on AMO reserves or other options into the PID.
Theirs two blueprints, a spread out and annotated version on a bare bones space platform if you want to see how it works, and a small demo ship where I squashed everything together to save space.
The demo ship can get to the 4 inner planets if you have shooting speed and physical projectile damage up to before they become infinite researches.