It's more correct to say that water mixing valves OPEN when the hot or cold solenoiods in them are energized. That's because their default position is closed, so they remain closed unless and until their solenoids are energized, allowing pressure in the water supply hoses to open them.

The water hammer that can happen when a washing machine, dish washer or toilet tank stops filling is due to the fact that the diaphragm in each valve closes very quickly. It's because of this that you typically see surge arrestors on the water supply piping going to washing machines.

Back when they used to sell overhaul kits for washing machine water mixing valves, I overhauled the water mixing valve on my main floor washer. The hot and cold springs that push the plug back into the diaphragm when the coil is de-energized aren't any bigger or stronger than the springs you typically find in an inexpensive ball point pen.

This image explains how the water mixing valve in a washer works fairly well:



Water comes in through the hoses and filtration screens on the right hand side.

The rubber diaphragm rests on a plastic seat. Water flows through that seat to get into the washing machine wash basket.

The water pressure is the same on both sides of the diapragm, but the area occupied by the seat reduces the area over which that pressure is applied on the far side of the diaphragm.

So, as long as the machine is off, the pressure equalizes on both sides of the diaphragm, but since force = pressure X area, there's a net force pushing the diaphragm down tightly onto the seat so that water can't flow into the washer.

When someone starts the washer, the hot, cold or both hot and cold solenoids are energized. This pulls the brass plated iron plug out of the hole in the middle of the rubber diapragm. When that happens, the pressure on the large area side of the diaphragm is released and the city water supply pressure on the small area side of the diaphragm pushes the diaphragm off it's seat. Water then flows under the diaphragm into the washing machine.

When the pressure switch in the washing machine senses that the washer is full, it stops the flow of electrical power to the hot and cold solenoids so that they are no longer pulling the brass coated iron plug in. The spring then pushes the brass coated iron plug out so that it covers the hole in the middle of the rubber diaphragm.

The water pressure then equalizes on both sides of the rubber diaphragm, and because the area of the seat reduces the area exposed to that pressure on one side of the diaphragm, there's a net force pushing the diaphragm down onto the seat so that water flow into the washer stops.

Here's what a water mixing valve for a washing machine typically looks like:



The two supply hoses you connect to your washer actually screw on to the mixing valve itself. Each solenoid is energized depending on whether you set the machine for a hot, cold or warm wash. The water enters at the two male threaded hose connections and leaves via the smaller tube on the opposite side of the valve.

Here's the water valve for a dish washer:



There's only one solenoid because you only supply hot water to your dish washer.

And, a toilet ballcock works exactly the same way, except that it uses the movement of the float to pull the plug out of the rubber diaphragm when you want water to flow into the tank, and push it back into the rubber diaphragm when you want to stop filling the toilet tank.

There you have the dirt on toilet ballcocks.