https://youtu.be/l789l6np-qA
It's the way he drags or pushes the chalk.
Angling it in the opposite way to normal makes it 'jump', creating the dots.
Indeed. Similar to a drum-roll. Or, probably more accurately, making that noise by rubbing your finger round a wine glass rim.
@Soul started
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https://youtu.be/l789l6np-qA