A generator interlock kit (or just interlock kit) is a device designed to allow safe powering of a home by a portable generator during a power outage.
It is a less-expensive alternative to purchasing and installing a dedicated transfer switch.
Backfeeding the external mains is unsafe and illegal, because it can potentially electrocute lineworkers, start fires, and overload or damage the generator.
[1] Early generator interlock kits consisted of two sliding steel or plastic (depending on the brand) plates held together by three bolts and installed on the front cover of the home's breaker panel, however, some models made by Eaton (formerly Cutler-Hammer) and Siemens for panels manufactured by them install on the adjacent circuit breakers themselves and consist of a sliding arm for breakers installed back-to-back or a pivoting arm for breakers that are installed side-by-side or one above the other, these arrangements remain on the breakers even if the panel cover is removed as opposed to the mechanisms installed on the panel covers.
A short cord connects the generator to the house inlet, usually through a twistlock plug and socket.