That is EVERYONE's fear of going clipless.
Seriously - once you get the "muscle memory" working, it's like simply stepping off the pedals.
I always suggest for first timers...go to a wide open, grassy field. Ride around - come to a stop and practice unclipping for at LEAST an hour. The key is coming to a STOP and working on that natural "unclip". I ride pretty hard core technical trails and always ride with my Times.