These are USA toilets. YMMV.
In the bottom of the bowl, there is a big hole in the back into which flushed debris vanishes.
In the front of the bottom of the bowl is a 1" diameter hole.
What the hell is it for?
water comes rapidly from the tank into the start of the siphon trap of the toilet, that starts the flow quickly causing agitation which helps keep a turd blockage from occurring. more successful flushes with less overflows.
From the 1" opening? It looks as though it comes from under the rim of the bowl.
Both,
The small hole acts as the inlet to a venturi. It increases the water velocity through the trap and reduces the pressure relative to the toilet bowl. If you want to see the difference, put a cork in the hole and see how much slower the toilet drains.
I just reset a toilet, so I feel qualified to comment on this! 
Yes, it’s a jet that helps propel poop down the pipe. I can say with experience that it won’t help a roll of medical adhesive tape get down the pipe, though.
You know how you used to have to pop two holes in the top of a can of beer? Whatever “effect” that is, that’s it. lol
Pulling a toilet gets easier with experience. I’m afraid I’m quite good at it.
Thanks everyone for your answers.
(bolding mine)
Just a WAG, but I’m betting there is a small child (or two) in YOUR house. 
No, but there were in the rent house. I just dropped and broke the handle off a coffee mug. Got tired if it rattling around whenever we flushed.