Not quite the same, but one of the math professors from my alma mater could write faster on a blackboard than most people could write with pencil and paper and was not shy about erasing when he filled up the board. It was always crazy trying to keep up with him.
I remember a prof, I think math, might have been physics (there was one guy in the physics department whose MSc was in mathematical physics, was working on his PhD, taught in both departments, and seems to my shaky memory to be the best match for what happened) who actually had the eraser in the "other hand" one day, applying it as he went. I think he was just excited, cannot remember why. One or more us said something like "uh, professor...?", he noticed, put down the eraser, and continued, somewhat sheepishly.