I’ve never heard of this pressure control
Pressure-balanced valve - https://en.wikipedia.org/wiki/Pressure-balanced_valve
> A pressure-balanced valve provides water at nearly constant temperature to a shower or bathtub, despite pressure fluctuations in either the hot or cold supply lines.
thermostatic showers - https://www.steamshowerparts.co.uk/how-to/thermostatic-showe...
> A thermostatic shower seems like a regular shower, but it has something very unique inside; a thermostatic valve. Thermostatic valves mix both hot and cold water together to a consistent predetermined temperature, preventing scalding and thermic shock.
left-right controls the temperature, up-down controls the flux
The smaller knob controls if the water goes to the showerhead or just out the faucet
https://www.kludirak.com/pub/media/catalog/product/cache/b3b...
Just picture a one-handle kitchen faucet with a long hose attached. At the end of the hose is the shower head.
It’s much easier to convert an old tub faucet to a shower this way, than by opening up the wall to run a pipe up to a fixed shower head.