The Red handled ball valve controls the flow to the DekClor. The more water flow into the DekClor, the faster the tabs will dissolve.
I always have water flowing thru mine - even though I don't use tabs any longer.
We recommend liquid chlorine over tablets. Tablets add CYA (stabilizer). Being in AZ (like me) you will go thru a lot of tabs and the CYA will build with each tab. Many of us have switched to SWG as, in the long run, the SWG is cheaper than other chlorination forms.