It uses a Pressure/Vacuum system. The propeller is more or less the pump impeller. Usually the start position is fast enough for the prop the force water into the water intake on the front of the exhaust outlet. After the water jacket is full, the water runs dawn the outlet tube to an opening in the back of the gear case in front of the propeller. This acts as a siphon helping pull water up the intake tube. Once this circulation is established, the PO will remain cool at an idle. Any air leaks will defeat the system and cause overheating. So – a few seconds running dry will do no harm. . . 🙂

I test started mine this summer using a "fountain pump",
sucking water out of a 5 gallon bucket. I had the prop
off. The fountain pump was plumbed to the "water intake"
using a block of wood and rubber gasket and a
Rube Goldberg affair to clamp it on.
It did work, but I had a hard time getting a good
seal, so I only ran it a couple of minutes.
It had potential, but a better design needed!