The conductive and resistive properties of water are employed to carry electric current.
If more pressure is needed (more steam) the controls speed up the pump to increase flow through additional nozzles.
As the needed pressure is reached the pump controls the flow of water to obtain the desired steam output (in unit mass per unit time) at the desired pressure.
On larger systems the pump can be controlled by a variable frequency drive so energy is not wasted.
When evaporated into steam, deionized or distilled water leaves little or no ions in the boiler respectively.