From GTPlanet Game Guides

Real time Adjustment is a feature included in Gran Turismo 5: Prologue, it allows you to alter settings during racing, such as Traction Control setting (TCS), Active Stability Control (ASM), Brake Balance one the front and rear, (in certain cars) 4 Wheel Drive (4WD) on/off, and Anti-Lock Braking System (ABS).

How to use

Accessing the feature is disabled unless the user alters their controls in Gran Turismo 5: Prologue's settings menu (Note, if the user has a Logitech Driving Force GT, it is automatically set up for use of the Real Time adjustment menu)