If the engine is controlled via cable (drive by cable) directly from the gas pedal, then there is no difference if you were to open the throttle by hand and open the throttle plates. In fuel injected cars of this type, there is usually a throttle position sensor which tells the computer how far the plates are open and adjusts the other engine components accordingly.
The caveat I'll give you though, is that most vehicles today are drive by wire, meaning, there is a rheostat attached to the gas pedal which interprets your intent as to how fast the engine should be going. This signal is fed into the ECU, which in turn opens the throttle plates and adjusts the engine accordingly. This doesn't mean the throttle plates operate exactly as you push or release the pedal. Since it's controlled by the ECU, the ECU decides when and how far the throttle should open or close. The throttle usually does follow pretty much what you are telling the gas pedal, but that doesn't mean it does it exactly. The other thing about it is, with drive by wire systems, there's nothing to turn or grab hold of with the throttle plates. There's a motor which turns the plates and is self contained. You cannot poke your head under the hood, grab hold of the throttle, and give it a whirl. It just isn't possible.