When the door is closed, a connection inside the lock powers an electrical coil wrapped around the bimetal part. This heats up the metal, which then bends. This bending operates the lock and then makes another electrical connection, passing power to the rest of the machine. The switch is located behind the front panel. If the switch has shorted out and is not locking, it is no longer delivering the message to the machine, and it will not latch into place. The thermal lock is preventing the door from opening. It works by heating up and expanding, so even when the washing machine is switched off, the door won't unlock immediately.