This is my skimmer, the valve on the right has me a bit puzzled. The black hose is from the filter, the grey is to the return.

In it's current position, let's call it up going with the pointy part of the valve-stem, the return flow is normal.
To the left it starts some kind of waterfall thing, there's no weir or place to attach one as far as I can see.
Down and the return shuts off, is this a backwash setting? <-- that's my first question.
My other question is about the waterfall thing, does that serve the same purpose as the weir door?

Or am I totally clueless?

I've spent so much time cleaning the water that now that that's about done I can learn the rest of the system.