Hmm... Car and Driver says "to boost midrange power in gears three through six". However the description in the ST supplement says improved performance for passing and vehicle launch. You're not going to launch in third gear. at least not on purpose
Your ST vehicle offers a period of
additional torque delivery referred to as
overboost or overtorque. This feature of
the engine calibration broadens the RPM
range of the peak torque curve. This gives
improved performance during maneuvers
such as passing and vehicle launch.