Originally Posted by goldminer
Most turbocharged engines have a thing called a wastegate after the turbo. The wastegate is a dump valve which bleeds off overpressure in the intake system. In modern cars it is usually under computer control. The overboost feature will likley also be under the control of the engine management system. It will allow the wastegate to open at the higher pressure for however many seconds it is set for, then revert to normal pressure. This feature will proably work only at or near full throttle. You need do nothing other than put your foot all the way down.
This is the what I would believe too. I know Porsche's turbo gets overboost when and only when the go pedal is floored and then for 10 seconds. Although overall hp does not rise with over-boost it is very likely that the mid-range hp (not only torque) rises as well. Over-boost for FI engines is a great way for manufacturers to get quick spurts of additional power without exceeding reliability.