OLDTUX, Technically, you are always "on the turbo". In other words, some amount of boost is always present.
GM uses a twin scroll turbo. The twin scroll design gives the best turbo output for all-round operation while at the same time limiting turbo lag.
In effect, you have two turbos in one. You have the low speed boost response of a small diameter turbo yet you have the high volume boost output that you would normally find in a big diameter turbo.
Boost pressure is continually monitored and controlled by the computer and any excess boost pressure is dumped by computer controlled wastegate.
Think of it this way. If the turbo was not (always) producing adequate boost, and you jammed the throttle wide open, the engine manifold pressure would go very low, maybe into negative pressure. The engine would immediately bog down until the boost pressure rose to an adequate level. That is what's called turbo lag.