Overriding is the ability to control machines. While an overridden machine cannot be commanded per se, overriding turns a machine from its default state of hostility toward any human to a state of combat friendliness toward the human who overrode it, in that instead of attacking the overrider, it will attack other machines or humans that attack the overrider.
The Nora Brave Aloy gained the ability to override machines after defeating the Corruptor that attacked Mother's Heart in the aftermath of the Proving Massacre. Upon examining its chassis, she discovered the component that Corruptors used to slave the CPUs of other machines. She attached the component to her spear and began to use it to override machines. At first, she was only able to override the weakest machines: Watchers, Striders, Broadheads and Chargers. However, after downloading override codes from successive Cauldrons into the component, she gained the ability to override successively stronger and more dangerous ones, culminating in the strongest and most dangerous machines: Thunderjaws, Stormbirds, Behemoths, and Rockbreakers.
The wandering researcher Sylens learned about the override ability by watching Aloy through her Focus. Presumably, he was able to advance the skill to actually being able to command machines, as he was able to bring three overridden Ravagers with him via a machine lure attached to his lance to aid in his rescue of Aloy from the Eclipse leader Helis after he captured her.
It appears that overridden machines are immune to corruption, since the two Corruptors that Helis had commanded to kill Aloy were helpless to stop the three Ravagers from attacking and destroying them; they would doubtless have corrupted the Ravagers and had them help to pursue the escaping Aloy and Sylens if they could have done so. The implied immunity of overridden machines to corruption is logical given the use of the Corruptors' own slaving technology in the overriding process; the Corruptors would have been essentially slaving CPUs that were already slaved by their own technology, which is impossible. Likewise, Corrupted machines cannot be overridden for the same reason: it is not possible for the slaving technology to be used against itself.
There are six classes of override, each allowing the ability to override a different group of Machines.
These are automatically obtained by Aloy when she searches the carcass of the defeated Corruptor at the end of the main quest, The Womb of the Mountain. The cauldron itself cannot be visited in-game. They grant the ability to override the following machine types:-
These are obtained by completing Cauldron SIGMA, and grant the ability to override the following machine types:-
These are obtained by completing Cauldron RHO, and grant the ability to override the following machine types:-
These are obtained by completing Cauldron XI, and grant the ability to override the following machine types:-
These are obtained by completing Cauldron ZETA, and grant the ability to override the following machine types:-
During the main quest The Mountain That Fell, Aloy will discover the master override in GAIA Prime, which is essential in defeating HADES as it can purge the Extinction Protocol. The master override is then fitted near the blade of Sylens' Lance.
The ability cannot be used on an attacking machine, nor can it be used on corrupted machines. In order to be successfully used, the machine must be either unaware of the overrider's presence or physically restrained, e.g. by Ropecaster lines. Furthermore, damaging the overridden machine above a certain threshold will revert it to its default hostile state.
Mountable machines are always overriden indefinitely, but other machines will only remain overridden for a limited time. The override duration for non-mountable machines can be extended by 50% using the Combat Override skill, or indefinitely with Combat Override+.