How it works? Equipotential bonding is applied in a bathroom to keep all exposed metalwork and earth conductors at the same potential (voltage) - hopefully zero. Then if you touch the shower and the radiator at the same time, for example, there is no chance of a shock by current travelling through yourself. If, for example, the radiator weren't bonded and there was a floating voltage on it, and you touched it and the bath taps which are earthed, you would be acting as the earth conductor, which is a bad thing - it hurts.
If a fault does occur, making any metalwork live, it has a safe route back to earth through the lighting earth and the shower's earth, and probably a main bonding conductor too. This easy route to earth makes a large current flow and trips the MCB / fuse of whatever is causing the fault.
Thats the basic principle how i understand it anyway.
