The quick start guide states "The motors can only be stopped mid-flight when the flight controller detects critical error." I don't what what that error is but you probably can't just stop it while flying, but it would definitely be interesting to know if that's true or not.
You can enable it to stop anytime in the settings. I doubt they'd recommend it for descent, however I've seen a phantom video where someone accidentally did that over the ocean and managed to save it just in time.