The pressure valves or poppet valves also control water flow but do so from pressure instead of temperature. In the picture you can see the springs will be going into the power head and hold constant pressure on the poppets and they seat into the rubber grommet. When you get into the higher rpm's like at plane speed the water pressure pushes these open and floods the power head.
Close but backwards. The springs aren't in the powerhead. They are in the adapter plate which is where the water escapes. The power head is always flooded, but flow is limited by the thermostats and poppet valves.
