Heating the water creates vapour pressure, which pushes the water through the filter filled with coffee powder and the riser tube up into the pot. The exact pressure at which this happens depends on several factors such as the grind of the coffee powder and the filling quantity of the filter. As pressure and temperature are correlated (higher pressure = higher temperature), these factors can also influence the flavour and consistency of the finished coffee. If the ‘resistance’ from the coffee powder is too great (e.g. if the powder is too fine), the pressure will be too high.
A pressure of approx. 1.5 bar is ideal, as this is the perfect extraction time to release all the flavours.