A device that allows Ancients to lock a city-ship and its systems out of anyone else's control. In Atlantis, it is stored directly beneath the front of the Stargate and will quickly rise through a concealed sliding hatch under the gate room floor when triggered. Any authorized personnel immediately arriving can, without warning, call upon and utilize the unit to lockdown the city.
As part of its function, it instantly disconnects any foreign devices (such as non-Ancient computers and monitors) which are interfaced with the city, and reactivates the Ancient displays and control systems. It also revokes any system administration privileges created by the city's present occupants and grants command of the city instantly to its user. It appears to activate via neural command, despite the user having no physical contact with any part of the city. This would make it the second known Ancient device to be able to interface with a user's mind despite not being in physical contact, the other being the Long-range communication stones. (SGA: "The Return, Part 1")
It is unknown whether or not the device is only able to be activated by certain high-ranking officials, or if it is configured to read the Ancient Technology Activation gene itself; or perhaps one of the many other genetic differences between baseline humans and Ancients. Unless put into use, control of the city falls to whoever manages to occupy it themselves.
It is unknown whether all city-ships possess this safeguard, or just Atlantis. If all city-ships do possess it, then it is likely that the device is located in the control tower on every ship. Judging by the importance of its function, it is probable that a similar lockout command can be called from a variety of places throughout the city, perhaps through control consoles or through further hidden devices.