Craig, i believe to achieve a star for a particular segment of the work out zwift would check whether you have hit the correct Wattage/power zone which would also include +/- Watts and for how long you have been in this zone. This is a similar approach to TrainerRoad and their workouts i have used (mainly sufferfest videos).
I have had to reduce the power by 5-8% whilst in the workout - just cant hit 470watts for 30 secs after the first couple of segments. I too failed to get a star for the recovery as it the kickr took ages to reduce its resistance down to the target recovery wattage.