As the rods exit the column at the lower end and connect up to the pedal bars, they all bend outward in order to reach the connection point on the pedal bar. The ‘diaper’ is there to support the rods through that bend so that they don’t flex every time you move a pedal. It does that by pushing the curved area of the rod against the outside wall of the hole in the column. The felt diaper also helps to prevent noise or vibration of the rods when the instrument is being played.