A kill switch, sometimes called an emergency stop, is a part of a user interface. User interfaces allow people to operate machines. The kill switch is there for security. Its purpose is to switch off the machine as quickly as possible, or to bring it into a state where it can no longer do harm. It is used in an emergency. In many ways, switching off the machine in an emergency is different from switching it off normally, as it has to avoid injury or harm at all costs.