That one controls which pieces of the seat lock together at that level. I have been on a flight where that hadn't been done properly, and a piece of the seat kept flopping back into the lap of the poor pax behind. A quick lips of the head rest ear sorted that one out once an engineer could be found.

However, I think the main slide mechanism is underneath the seat. It's quite amazing to watch how quickly it can be done. I was on another flight where the curtain had been moved but the seats had not been reconfigured - there were a lot of unhappy B seat occupants. The seats got reconfigured in just a few minutes, without any tools.