I have been having a problem: Every time I find a funny patent, I find a whole set of related patents and it leads me down a rabbit hole. I never know where to stop. Prudence dictates that I should spread them out, so that I have content for years to come.
But I can’t help myself. So here are four machines with a shared purpose.
The first is the “user-operated amusement apparatus for kicking the user’s buttocks”: https://patents.google.com/patent/US6293874B1

Res ipsa loquitur
This is the patent that started my unfortunate journey. Note the label, “B.” Note the arch of the back. Note the snazzy shoes. Who decided to use those particular shoes? Maybe I’m just uncomfortable because I have owned shoes like that, and now they’re ruined for me.
In accordance with the present invention, an amusement apparatus is provided that includes a user-operated and controlled apparatus for self-infliction of repetitive blows to the user’s buttocks including a plurality of rotating arms bearing flexible extensions for self-paddling the user’s buttocks B.
USP 6,293,874, col. 2, lns 54-69
The ‘874 patent was courteous enough to spell out the state of the art in self-spanking, citing a 1909 patent, the “combined lifting and spanking machine”: https://patents.google.com/patent/US920837A

This one “related to the art of initiation devices,” presumably to be used in some ominous secret society where the children of plutocrats haze one another for a while before going on to govern nations.
It is of course to be understood that the mechanism concealed beneath the platform can be readily used in connection with other forms of trick devices so as to subject the initiate to an electric shock.
USP 920,837, p. 2, lns 75-79
“of course”
Further explorations of this specialized field of technology uncovered the “manually self-operated butt-kicking machine”: https://patents.google.com/patent/US20060094518A1

This patent application was never granted. Its broad claims were found to lack novelty and were rejected. The inventor could probably have fought that, but maybe he had sobered up by then.
The butt-kicking operation can be administered continuously or intermittently as determined by the user or the operator. After each kicking operation, a spring moves the lever back to its original position, so that the next round of butt kicking can proceed. … The butt-kicking operation is continued until the desired results are achieved.
USPP 2006/0094518, ¶12
What was the prior art, you might ask, that stopped so brilliant an invention? It was the most straight-faced of the bunch, the “physiotherapy and health improvement instrument”: https://patents.google.com/patent/US5904660A

That’s the face of someone
in a training montage
Now, let me lead by saying that prostate health is a very serious matter, and I’m sure Mr. Kim was doing his level best to promote male wellness. Just please don’t try this at home, okay?
As physical treatments available for suppressing the enlargement of the prostate gland, there have been used physical treatments, such as repeated impacts on or frictional stimuli to the prostate gland site.
USP 5,904,660, col. 1, lns 14-17
I wish I had a good explanation for why this thing has been invented so many times. Filing a patent application is a lot of effort just to make a butt joke, so I think these people sincerely believed there was a market for their devices. And maybe Mr. Kim did think there was real good to be had from repeated blows to the perineum. And you never can discount the possibility that it was all some kink thing…
I don’t think there is a simple answer. I’m just going to chalk this one up to the unintended consequences of telling children to follow their dreams.