It is not always clear why, but when the mind is thinking, not everyone’s body is at a complete rest. Especially their fingers, even if ever other part of the body is calm. Some people shake their legs, some people turn their chairs letf and right and some people play with their pencils or scribble something on their blotters.
If the body is completely idle, it means tha tyou are either sleeping or meditating. And wen your body is dong somethhing, leaving yourbody idle might make you get into sleep, which could be bad for the thought process. So while the mind is thinking, it is important to give thebody to do somehting so that it wont bug the mind while its busy. So the mind naturally asks the body to go play with the pencil for a while while it thinks. Well, to keep the body more occupied, people could do better things that doesnt need the mind’s assistance much, such as toss a yoyo and catch it back, which will exercise your cerebellum too, as well as let you go on with your thoughs and planning processes without much disturbance.