Very hygiene sensitive...
Here are the "buttons" you see in the relatively modern buildings - rest rooms for the public.
What is the difference?
On the left - you have to "press" the button to flush.
On the right - you just have to put your hand "close" to the panel.
Right hand side is more common in the newer buildings.
Simple. When you "press" the button, you "touch" it. When you "touch" it, you don't know what you actually "touch" and bring with you. But when you just get your hand "close to the panel", you don't touch anything, so you would less likely bring "something" with you from the rest room.
Very clean country, indeed.....
Just a short tip for the toilet use in Japan...