The following image illustrates the tripping mechanism of the MCB. The circuit breaker contacts and the position of the knob can easily be seen in both, ON and OFF, state. In the ON state the moveable contact touches the fixed contact as shown in the image.
The path of the current in the ON state is shown in the image below. The live wire is screwed at the input terminal. From the input terminal the current flows to the fixed contact which is touching the input terminal. The current then moves to the moving contact (since in the ON state it is touching the fixed contact). The moving contact is connected to the electromagnetic coil of the solenoid by means of a thick wire. The current from the moving contacts enters the electromagnetic coil. The current from the coil goes to the bimetallic strip by means of another thick wire and finally to the output terminal where it is collected by the neutral wire of the line.