On my old MX Revolution, middle click was the switch between it indexing and free-wheeling...and also sometimes registered as a click/key, e.g. - I played games

On my old MX Revolution, middle click was the switch between it indexing and free-wheeling...and also sometimes registered as a click/key, e.g. - I played games when I had it - 'stab with knife' in the Battlefield of the day; more obviously it was also the zoom in scopes that supported it, so I'd find myself stabbing the air to get it in the right mode when trying to use it as a sniper...