What does the black xxxx under the chair control?
Cosset Mid-Back Executive Chair, Supports Up to 275 lb, 18.5" to 21.5" Seat Height, Black Seat/Back, Slate BaseChairs & Stools
According to the manufacturer, the xxxx on the bottom of the seat cushion controls the tension that allows the chair to lean backwards. If turned left, it will loosen the pressure. If turned right, it will tighten the pressure and therefore make it harder to lean backwards.