A bi-metal strip is the heart of a circuit breaker. The first understanding is that when a metal is heated, it expands. Different metals expand at different rates. For example aluminum expands faster that iron. A bi metal strip is made of two metals placed one on top of the other. When it heats, because metals expands at different rates, it actually bends when heated. In a circuit breaker, a bi-metal strip is placed across two contacts. When heated up because of an over-current, it bends away from one contact, breaking the circuit.
Two contact points are placed inside a breaker. These are usually flat round metal pads, with the bi-metal strip placed across them. The strip is attached firmly to one, but only sits on top of the other. When the bi-metal strip heats up, it bends and pulls away from the non-attached contact, breaking the circuit.
A small spring is strategically attached to the bi-metal strip and the trip lever. When the bi-metal strip bends, it pushes on the spring. The spring pushes on the trip lever, making it go from the "on" to the "off" position. The spring also holds the bi-metal strip down, so it sits firmly on the non-attached contact. At the same time however, it has enough "give" so the strip can pull away from the contact when heated.
A circuit breaker is like a fuse, except that it is reusable. A trip lever makes it reusable. When the bi-metal strip pushes on the spring, the spring pushes on the lever. When the strip cools down, the spring leaves the strip in the open position. The trip lever , much like an on-off light switch on the wall, is pushed over to reset the bi-metal strip and the spring from "off" to "on." Inventor Joseph Fierro invented a breaker in 1989 that utilizes a new type of trip lever mechanism to move the strip when the lever is pushed.