Unlike a simple passive Group Valve, the Three-Way Steam Solenoid Valve is electrically actuated, typically by way of an electromagnet pulling or pushing a metal piston up or down a tube. This allows the valve to selectively allow water to pass between two of its three connections, blocking flow in and out of the third. Most commonly, this produces the following conditions:
Allow water to flow between The Brew Head and the Drain Port, blocking off the Boiler outlet.
Allow water to flow between the Boiler to the Brew Head, closing off the Drain Port
When espresso machines equipped with a Three-Way Valve are sitting idle or unpowered, this valve sits in the first state; this allows water and air and water to flow freely between the brew head and the drip tray while keeping water neatly contained in the boiler. This keeps the brew head dry.
Toggling the brew button feeds power through the valve's electromagnet, switching it into its operational state; The drain port is sealed shut by the valve's piston. Generally speaking, the pump is powered on as well to force water into the boiler although some manufacturers have added a delay circuit to this process to allow for pre-infusion to occur.