Why is it when your taking a shower and someone flushes the toilet you lose all water pressure?

Imagine a hose pipe in the garden. When the water is on and there is nothing attached to the working end, the water comes out in a steady pour. Now, if you put your thumb over the end suddenly the water starts to squirt out under pressure from beneath your thumb. This is because as you constrain the flow the mains pressure behind the water mounts up to its maximum value while with the unobstructed hose there is no pressure because the water is moving out of the pipe at an equilibrium rate.

Now, if you think of the shower, it constrains the flow of the water, rather like your thumb over the hose end and when someone flushes the toilet this creates another opening in the pipe coming to your house and the pressure goes away like the unconstrained hose end.

To avoid this problem many people supply their shower from a header tank in the roof which is filled from the mains separately and provides a constant dedicated supply to the hot water system and the cold side of the shower so that the pressures in each balance.