Locate the standard size Star Destroyer near the top center and compare to the World Devastator to the right. Ya know, when the Empire lands on an idea, they stick with it (I'm talking about you, Death Star). This is how Palpatine's Great Sith Fleet was stopped in Episode IX.
This is how the droid Army was disabled in The Phantom Menace.
This is how Luke Skywalker and R2-D2 deactivated all of the devastators at the same time for the Alliance. The Devastators were hard coded to shut down should the massive droid loose contact. As long as a World Devastator was in contact with Byss via this signal, the great machines worked. The signal constantly beamed from Byss throughout the galaxy. The World Devastators were in constant contact with Byss, the Emperor's stronghold, via a coded and secreted hyperwave signal. For the World Devastators, there was a safeguard against the droid brains being re-programmed and used against the Empire. Another thing from Dark Empire that must have been stolen for Skywalker Rises is the master control signal.
