Think of the water flow thru the pump. One direction on the pump closes the out valve and opens the in valve, the pump cavity fills. The next stroke closes the in valve and opens the out valve and pushes the water up the line. So when there is pressure on the in-side of the pump, the valves will allow thru-flow. Hope this makes sense
So, with most hand pumps if you install on the pressure side of the system you'll need a stop **** to stop water being pumped through it.
This is why it should be plumbed into the non-pressure side of the electric pump.