Here he adapts an ordinary 40 amp car relay, converting it from having a “normally open” contact, to operating with a “normally closed” contact. It is not necessary for you to do this as automotive relays with “normally closed” contacts are readily available and are not expensive. The relay is then wired up so that it powers itself through its own contacts. This causes a current to flow through the relay coil winding, operating the contact and opening it. This cuts off the current through the relay’s own coil, causing the contacts to close again and the process starts all over again.