I think he means the type where your hand manipulates the ball, not a mouse with a ball.

This got me thinking. A modern ball mouse utilizing some kind of optical tracking on the ball could be very accurate. Of course, probably less accurate than tracking on the desk itself, but would be fun as a "retro-style" novelty.

I do believe Microsoft and Logitech have built mice that had optical tracking of the ball but went to direct tracking of the surface because of cost, cleaning, and accuracy.

That's how modern trackballs work. Optical mice are cheaper to produce and more reliable than those with balls, and trackballs are the same in that regard.

I use a trackball so I don’t have to move my arm or stress my wrist. I use a Logitech trackball that only requires moving my thumb.

I love my old-school TrackMan Marble Wheel. It's even better than the modern Logitech trackball.

That's the one I use. I even bought some extra since I hear the wireless isn't quite as crisp.

When I notice an onset of RSI I switch to a thumb operated trackball for a couple of days. Also when I produce music and haven't enough space left besides my controllers on my desk for operating a conventional mouse.

If you're having to switch to hold it off it might actually be being caused by another ergonomic problem with your space.

I kept having to change things/switch out things to hold it back until I one day adjusted my chair/armrest height to above my desk and I haven't had an issue at all in 6 years since doing that.

I use a trackball because it takes up less desk space and doesn't need a mouse pad.