A restraining bolt, also known as a restraining separator bolt, was a small device that could be attached to most droids used to ensure their obedience. More specifically, when fitted with a restraining bolt, droids could not leave the area designated by their owner, and were forced to obey commands given from a small remote device called a caller (such as "COME" and "STOP"). When activated, the restraining bolt turned off a droid's motor impulse without actually shutting down the droid. Restraining bolts had to be secured to specific locations on the droid.
Restraining bolts could also be used to shut down a droid, and power it back up again. This could be used to force a restart if there was an error in the droid's memory. The shut down could be accomplished with a switch on the bolt, or through the use of the caller. Kell Tainer helped Myn Donos restart his droid Shiner using this method after Empion mines had disabled the squadron.
The astromech droid R2-D2 tricked his new owner Luke Skywalker into removing his restraining bolt, so that he could leave the Lars homestead and seek out Obi-Wan Kenobi. R2-D2 and C-3PO were also fitted with restraining bolts during the Bakuran Crisis. The assassin/protocol droid HK-47 also had a restraining bolt, so that he could not leave the droid shop until he was purchased and collected. Whistler, Corran Horn's R2 unit, had a unique mechanism which shifted control points internally, allowing the droid to avoid being controlled by restraining bolts at all. EV-9D9 also modified herself so as to be immune to restraining bolts, and (for obvious reasons) security droids such as the G-2RD could not be disabled by standard restraining bolts.