Sidenote but what's that strange mod he has on his Herman Miller Aeron chair? The circle thing above the wheels? It's the first time I ever see that, despite ha

Sidenote but what's that strange mod he has on his Herman Miller Aeron chair? The circle thing above the wheels? It's the first time I ever see that, despite having seen hundreds if not thousands of Aeron in series, movies, pictures, at people's place etc. I dig it for I always put my feet on, well, the chair's "feet". EDIT: ah! Conveniently enough it's called a "foot rest". But it's not clear if it's adaptable on a regular Aeron chair or if it's only for the "stool" model.