if you are on a trainer and the wheel is coasting (i.e. the hub is making noise) then there is no pull on the chain, hence no torque, and the auto zero will work. If you are on rollers doing a track stand then it is possible (though unlikely) that you will "beat" the algorithm. Can you tell me in a bit more detail about how you coast? I have trouble picturing how you are coasting with torque on the cranks - sorry if I'm being slow.
The auto zero works when you coast at least 2 seconds (ie the system is unloaded for 2 seconds) and don't turn the cranks during that time. If you start pedaling again too early it won't zero. To be on the safe side I recommend waiting 5 seconds, then you are absolutely sure the zero is being made.
I hope this helps,
official power meter of Movistar Team