I learned this technique in the mid 70s, but I didn't attend MIT. I still remember a math class in 1973 that was sort of rowdy. One of the students was presenting. He wanted to draw a circle on the board, but there were actual, occasional, spitballs. If he took his eyes off of the class he would have been a sitting duck. Not to be deterred, this student, Kem, stood at the chalkboard facing the class with a piece of chalk behind his back, touching the board near the bottom. He swung his arm out and up and, at the same time, began to pivot around. His chalk went up to the top of the board, all the way around, and back to the bottom just as he turned to face the class again. The circle was elegant, no one had time to launch a spitball, and he gained another level of respect in the class - he already had a lot.
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