fas stands for "forced auto stop" basically engaging auto stop earlier than when the car comes to a complete stop.

the theory behind it is, the engine is killed and is not spinning therefore elminating coasting drag. They say the HCHII engine is designed so that it can seal its valves and then coast as good as a FAS however there hasn't been enough facts to back this up. Like PCK said a true FASER is needed to see if its truly useful on the hchII or not.

personally i use fas when i know i can maintain speed or bleed it off slowly for a long time, OR when i predict a traffic light is going to turn red on me. The theory behind it is, if you can coast up to a light and then by the time your required to stop, you only have maybe 10 mph or less in speed left which in theory is USELESS FOR REGEN. But the energy SPENT to get to that light is greatly reduced as compared to someone approaching the light at 30 mph and then using regen to slow down.

look at my tanks, it will speak what FAS is capable of, but i have a 5spd so i can turn on my engine with the clutch... the same way your electric motors give torque to your engines to turn it on.

i got around 20 km a liter when i drove a civic 5 spd using advanced hypermiler techniques
and now I drive a salvage nwh20 prius that gets almost to 25 km/l with 195,000 km to date.