THey work really well.
There is a spring that closes off the water flow when the output pressure is over the set point.
The problem is they also restrict water flow when the campground pressure is less than 50 PSI, so it is best to not use it at less than 50 PSI, many forest campgrounds are low to save energy on pumping costs. I bought a pressure gauge already hooked to a garden hose fitting at Home Depot last week, $12. I can measure the pressure, then use the regulator if needed.
Someone mentioned using a home style regulator. Because I was living in one area for well over a year, I bought a 3/4" regulator and fittings to adapt to my PVC water line. Then I was not limited to about 2-3 GPM that the garden hose regulators limit you to. Also from Home Depot, you can buy 3/4" Male pipe to female garden hose fitting and a 3/4" garden hose bib for the outlet so it will be portable and can be used at campsites. You will also need a 3-4' long drinking water hose, so you don't have to hang the 3/4" regulator from the garden hose bib, it might not support the weight well.
Fred.