It sends water through the whole system except for the pump and points upstream of it.
Also, sometimes when winterizing, people forget to get the water out of the city water fill line.
The check valve should not be too strong. Here is a Camco kit that just uses a hand pump.
http://www.campingworld.com/shopping/item/plastic-winterizing-hand-pump-kit/4411By my way of thinking, if doing it this way, you had better drain your pump first. Then reconnect and winterize.
Nice booklet from Camco:
http://www.camco.net/wp-content/uploads/2015/06/winterizeRV.pdf
