Did Hawkings turn off gravity?

lol...no. Astronauts use these planes for training too. All it is is the plane dives toward the ground at around the speed as gravity, 9.81 m/s^2, pulls. So the net effect is that you are in free-fall, or weightless for around 25 seconds before the plane pulls up. Think of it like a roller coaster...when you go over that first hill and drop really fast, you feel lighter. This works the same except that the roller coster is a plane and it goes alot faster!