If the Phantom detects it's not losing altitude anymore (aka it's on ground now), it will shut down after three seconds when you bottom out the left stick all the way for that long.
This won't happen mid-air though so don't worry. If you're a little weary about it, just throttle down like 90% max and it should come down without motors shutting off. :lol:
I know it won't shut off mid-air because while hand-catching it, it still detects the tiny altitude changes as my arms have trouble keeping still and hesitates to shut down.