Active Freewheel is basically just allowing the voltage in the ESC and motor to run in a circle until it's bleed off at partial throttle. It can keep your ESC cooler if you fly at partial throttle most of the time. However, if you notice surges, then you need to turn it off. If you can get it to work, then you can hopefully lower the heat produced. Basically, instead of the current passing through a diode, it will instead pass through another FET which has less voltage loss and thus, less heat.
It's complicated to explain, but turn it on and if you see surging, turn it off. Make sure it's not your GOV causing the surging (hunting).