This is a combination of the low pressure hot water system and high pressure water straight from the water mains at the street or directly from a pump.
Low pressure cold water supply to the hot water cylinder can also come by way of pressure reducing valves connected directly to the high pressure main. This system has its advantages and when used with a good unequal shower mixer you can enjoy the benefits of economy and a pleasant shower.
An unequal shower mixer usually has a venturi in it and the mixing of cold water with the hot water helps to pull the hot water to the shower rose and simulate a high pressure shower. Some single action tap mixers are specially made to mix low and high pressure water.
When ordering taps it is important to know you have this type of system and therefore will require special taps.