Originally Posted by NALEM

The placement of the litter/sand box on top of the toilet, gets the cats accustomed to visiting the bathroom and specifically jumping on top of the toilet seat.

Eventually some people switch out the regular box for something more shallow placed below the seat itself, or they place a cardboard panel there sprinkled with litter/sand.

As the cat focuses on balancing themselves on the seat itself, the shallow box or cardboard panel is completely removed.