Trackball Game Controller - Dismantled for reference and curiosity
The Trak-Ball controller while not being anywhere near as precise as a mouse, even a vintage one, was a lot of fun for use with games such as Missile Command. While this sort of title was the logical use, it was markets more broadly suggesting players would achieve higher scores from its '360 degree precision control'.
While logically its use it predominantly for games, the rear of the Trak-Ball's box shows both an Atari 2600 and an Atari 600XL computer as suggested compatible products.
It works much the same way as most ball mice, just upside down with the ball being on top. You move the ball and it rolls against two rollers that have little, flat discs on them called encoder wheels. When they turn the evenly spaced slots around the edge of the discs pass in front of light sensitive switches. Those X and Y movement light breaks are then sent as 'joystick' directions the 2600.
As it's such a nice, solid piece of hardware it's hard not to wonder whether Atari had envisaged more titles to make better use of it.