How were the slits through which the rods pass created?
One way of imagining their creation will be to begin by considering the different positions occupied by the rods.
We see that their track draws a surface in space, which called a hyperboloid of one sheet. On this surface two families of lines fit perfectly, precisely those which correspond to the different positions of the rods.
(*) This work was carried out under a grant by FCT - Fundação para a Ciência e a Tecnologia.