Bearing Nut, Child Safety Extensions
These rod extenders are a wonderful invention. They keep the end of the rod within the tube to protect people, mostly kids, from being struck by the rod. If you move your rod rapidly and strike a child in the head, it really hurts. These safety rods make that not happen.
They do not interfere with play at all. If you have small children around your table, you must buy these safety extensions. They take the place of a normal bearing nut.