A pressure water pump works with a pressure switch that activates the pump if the pressure drops. The pressure drops when you open a tap and immediately the pump will start and pressurize water. When the tap is closed, the pressure in the hose increases and the pump will stop.
If you want to be able to use a little water without necessarily activating the pump, it is advantageous to install an expansion tank on the system. Here, the water will be drained first, without the pump needing to start until the pressure has dropped.
