Power to the SWG should come off the load side of the power supply to the pump. This is for any type of circulating pump.

Not sure the type of switches you have-time clock, computer system-Jandy RS, Pentair.... On these types, get power from the load side of the relay. Always refer to the manufacturers wiring so you understand. Consider an electrician if your not familiar.

On a VS pump however, the SWG needs a certain flow rate for it to work properly. Refer to the SWG's manual for that. Adjust the pump's flow rate accordingly. Some SWG's have flow switches to activate them.

The salt generator has a power cwnter that controls it. It uses the flow sensors to only run when it has flow. So not sure why that is necessary ?

That's correct, the SWG has it's own power system. It can only function though when it is given proper water flow. The owner's manual should have that required flow rate listed. A VSP may have to be adjusted to activate the flow sensors on any SWG for them to work.

If a cell was powered continuously even when the pump was off, it would eventually damage the cell. There's no sense in relying completely on a SWG flow switch or sensor to turn the cell on or off. Sensors and flow switches will fail at some point. That is why the SWG should get it's power source from the load side to a filter pump to prevent any damage. This is for any pump type.

A dirty filter will cut water flow and not allow a SWG to be activated. Same holds true for a pool heater. It has a pressure switch that only gets activated upon water flow.

For our electronic auto fill devices, those too are powered off the load side to a pump. Pressure side cleaners with a separate booster pump also.