I thought you were a professional, and you don't know what a contactor is?
https://www.google.com/search?q=contactor+relay&tbm=isch
In a proper installation the pump never get directly controlled by the chlorinator, because the pump will draw too much current during start up and damage the chlorinator, hence the use of a contactor relay. Looks like the Thais know how to do it.
The pump doesn't get turned off because there is a flow issue, the chlorinator get turned off because there is a flow issue...........since the pump hasn't started when the chlorinator started.
A timer on a chlorinator doesn't have an auto setting either. You enter a start/stop time, or you don't.
The 3 way switches are obviously on the control box, one for each pump, so it can be operated manually, or by a timer or remote control, or switched off.