The upper left hand switch on the radio controller (TX) is for selecting flight mode. There are three positions. All the way up is GPS mode on. The center position is for ATTI. mode.
ATTI mode turns off the GPS location assist. The difference is in ATTI mode the Phantom stays at what ever height it is set at but when the forward / back or left / right controls go to neutral the phantom will continue to travel in what ever direction it was going in until an input is added from the pilot. In other words it acts like there are no automatic breaks that stop lateral travel.
So basically, you can be moving along in a direction and flip it into ATTI mode and it will continue to go in that direction. This is useful for following moving targets while you are filming.