Okay, I know I'm a newbie, but I'll chime in too.
What I've noticed is that the sound made by contact of the belt and steel tells you a lot about what kind of contact you're making. If I'm making contact along a long surface, the sound is more like a swishing sound, but if I'm hitting a smaller area it's more like a grinding sound. I can also tell by sound if I'm going over smooth surfaces or rough. I suspect the same would hold true for the grind stone in the Sheffield video.
The feel and sound are, for me, more helpful than sight. I have too many layers of protection between my eyes and the work anyway, making it hard for me to see what's going on under the best of circumstances. Even when only wearing glasses and a mask my breath inevitably fogs the glasses. In the Sheffield video the work conditions seemed pretty dark, so they probably wouldn't be able to rely on sight either.
- Greg