How do RV pumps work?

I got one and tested it, and found out how they're supposed to work.

You hook one end into a tank of cold water (like a spa or RV water tank). Then you hook the other end into a faucet (which can be a garden hose). You power it up.

If the outgoing garden hose is closed, nothing happens.

But if you OPEN the valve on the outgoing garden hose, then water starts pumping (by magic I guess) by the pump from the spa or water tank.

If you CLOSE the valve on the outgoing garden hose, then the pump (again by magic) senses that the output stopped so it shuts off the pressure pump.

The pumps don't even have on/off switches which is why it confused me. They go on and off depending on if water is flowing out or not.

Don't ask me how they know that water is flowing though.

I'd explain it but you probably wouldn't understand .

Thanks for the explanation that if the pump sees at least 43psi on the outlet side it shuts off, otherwise it pumps water from the inlet side.

The unit came with a cigarette-lighter socket also for use in vehicles.

That's kind of neat as it can also be used as a portable outdoor garden hose for watering plants which are far away from the house that you can truck the water to in ten gallon containers (or whatever size you carry).

Looking up what a "pony pump" is, I'm surprised I never heard of them.

"Pony Pumps are famous for their compact portable flexible impeller utility pumps and their versatile line of small submersible pumps. Pony pumps are non-submersible, self-priming nickel plated brass transfer pumps with Buna-N impellers. These lightweight, pumps can be used to empty waterbeds, drain water heaters or aquariums, pump out a basement, transfer water to an RV, pump boat bilges, pump out stock tanks, and in other applications where water needs to be transferred. Intermittent duty is 15 minutes ON and 45 minutes OFF. Two extra impellers and gaskets included." I wonder how they differ from the other liquid-transfer pumps though.

For example, this is billed as a 12VDC "self-priming diaphragm" pump.

And this is billed as a 12VDC "fuel transfer" pump. And this is billed as a 120VAC "water transfer" pump. And this is billed as a 120VAC "submersible water draining" pump. I'm not sure which pump I need for my application which is to have a solar-powered spa heater that pumps water in the summer from the spa through a few hundred feet of black hose back to the (now warmer) spa.