FADEC
Fadec is a full authority control for an engine measuring and matching aircraft requirements for flight, it normally has 2 independant channels for each engine (A & B channels) with independant power supplies and also independant of the airframe power supplies.
The fadecs normally take their references from the air data computers and flight directors for their calculations and output to the engines, and there is sometimes a switch in the cockpit for a mode selection (take-off, climb & cruise)
The pilot usually has a switch to allow him to select between channels for each engine but that's about all really, the fadecs usually recheck all parameters (flight requirements and engine parameters) about 40 times per second.
It would not be normal to have a pilot over-ride as the pilot's hand on the throttle would not normally be quick enough to control the fuel flow.
The throttle is a request really to the fadec to ask for more or less power than the fadec sees the aircraft telling it it needs, but depending on selection & configuration this isn't a guarrantee of changing parameters and varies between manufacturers, there are other external feeds into the fadec too like WOW sensors, flaps etc.
The fadec is normally at the heart of an aircraft avionic system as most other things feed into it and each engine and each fadec has an independant data source to ensure 100% redundancy so for 2 engines to fail at once is on a par with the big prize in the lottery after 10 months of roll overs.
There are some aircraft particularly turboprops where the engine has an HMU and this can have a complete manual reversion but I don't really know of any full fadec jets to have manual reversion although having said that I am willing to be educated otherwise.