"The order was programmed into the Grand Army clone troopers through the behavioral modification biochips implanted in their brains, making it impossible for the clones to disobey the command to turn against their Jedi Generals."
"Rex, receiving the command from Sidious, realized at that moment that Fives had been right about everything and managed to resist his chip long enough to tell Tano to 'find' the late ARC trooper before attacking her. After releasing Maul as a diversion, Tano was able to find Rex's grievance report, subsequently capturing Rex and removing his chip with the help of three astromech droids."