Why a shelf?
Here is an Austrian Toilet. It has a shelf. It is onto the shelf the poopy falls and gets smelly. Rather than falling into water, which creates somewhat of a smell proof shield between the poop and the pooper.
So what is the purpose of the shelf?
Not all Austrian Toilets have shelves. The modern potties have a nice little hole with a little water in it. Not too much water, like US toilets, but enough to give a person a sense of protection from the smell of his or her own poop.
If you know the answer to this mystery, please share