Not sure if it is a nervous habit or a ritual. I believe it started offf as a nervous habit. I keep a knife at my computer desk. I use it to drill and carve holes in the front edge of my desk. Each hole represents something I am working on or reading about. The size of the whole represents how much time I spend on each item. I might work on a whole for years because they grow very slowly. Making a whole for something suggests that this item has at least significant priority.
My wife would kill me if I drilled holes in our furniture.
The size of the whole will get reduced by the size of the holes.