What is this?

[anxiousatnight]

Anyone know what this square knob on my sink does? There's some sort of cable attaching it to the drain part of the sink underneath. It lifts up slightly but I can't get it to do anything.

We had a similar knob in an old house and it was supposed to raised the sink plug up and down rather than you sticking your hand in the water to pull it out. It never worked properly and shortly after we moved in stopped working at all.
could it be that?

Could it operate a waste disposal unit? We had one years ago with a blade that mulched all the leftovers to mush that then went down the plughole / drain.

I think it's probably what @PhotoTakenOnAToaster said, and ours doesn't work either. I might have a fiddle with it and see if I can get it going, I hate sticking my hand in the cold manky sink water!