Trapping of air – When you prepare your stove-top espresso maker for use, air gets trapped inside the water tank (A)
Pressure build-up – When heat is applied to the stove-top espresso maker, the air and water inside expand. As they are contained within the water tank, this leads to a build up of pressure in the tank. Water is pushed through from (A) to the filter basket where the coffee is contained (B)
Pressure release – Eventually, the pressure builds up to the point where the hot water is forced up from the filter into the upper chamber of the stove-top espresso maker (C). This is where the end result is stored ready to pour and drink