[Dave-ROR]

Originally Posted by LSxJunkie

Left button for Traction Control. Press it once and it'll let you spin the tires a bit but still try to keep you in a straight line. Hold it down and it'll step aside and let you try to kill yourself. Right button is for VSC Sport, which is, if I'm not mistaken, supposed to make you faster than your talent by letting you get the back of the car out more than normal.

See this part of the MT review:

Originally Posted by Motor Trend

Traction and stability control algorithms are governed by three buttons mounted just behind the shifter in 6AT-equipped BRZs. (Manuals lose the center Sport/Snow button.) With one touch, the left button partial turns off stability control; when held down for 3 seconds, it turns off completely. As you'd expect, the latter is a must for wannabe drifters.

Punching the rightmost button engages Sport VSC mode. This activates a combination of stability/sport indicator lights on the gauge cluster and lets you hang the car's tail out a touch, via steering angle, yaw rate, and lateral G sensors that forecast the vehicle's position. If that future looks too sideways, brake and throttle cut precautions engage. In practice, it's the best mode for spirited road courses like the SKC handling track. This twisty second/third-gear course had several tight low-speed turns and a couple of nasty mid-corner bumps that conspired to unsettle the BRZ. Sport VSC caught it every time, with a more progressive engagement of stability control over the normal mode.

Problem is it doesn't specify exactly what "partially turns off stability control" means... either way I'll have it off completely for track and autocross use, and on for the street as I don't drive agressively enough to engage it anyways and I really don't mind the nanny if something unexpectedly bad happens on the street.