There are 6 entries in the Guinea Pig film series (7 if you include the "best of" entry) and all of them would be suitable for "Beyond Nasty"; all are extreme gore / splatter movies, and all are nominally banned, never having been submitted to the BBFC.
However, we don't want to get bogged down in a 6 week run, so we've selected jut one of the movies to represent the series.
Part 2 was chosen, because it was actually investigated by Japanese authorities and the FBI as possibly a real snuff movie.
The only conclusion I can draw from this is that the aforementioned authorities are full if idiots, and that no one, at any time, thought to show the film to anyone who had even a rudimentary understanding if the human body, anyone who had ever seen real meat, or anyone who has any idea how cameras work.
The "story" such as it is, is that a manga artist received an 8mm snuff film from an obsessed fan, and that this movie is a dramatised recreation of that snuff film. This is explained in on-screen text, before the "dramatisation" proper commences. We then see a young woman kidnapped, drugged, and dismembered. If that sounds a little thin, then it probably explains the movies scant 40 minute run time.
So, from the on screen text we know that the movie isn't even TRYING to convince you that it's a real snuff film... Which is just as well.
Once the woman is heavily sedated, the first thing the killer does is to cut off her hand. It's a nice clean cut (he uses very sharp tools), and one he repeats on the other side.
Ignoring the fact that one of her hands seems to live for a while after removal (!) the blood barely trickles out of the artery (as opposed to the spurting one would expect) and she survives further amputation of the arms and legs, before being disembowelled; not dying until she is finally beheaded!
If the woman's ability to live with the kind of blood loss that would kill a mere mortal in under 60 seconds weren't enough to tip off the Feds that this was only a movie, the fact that her muscle tissue seems to be made of red jelly should have clenched it for them; add to that the fact that the camera angle changes regularly (but we never see any of the cameras) and the case on this one should have been closed in a single viewing.
But that's more a critique of the critical thinking skills of two nations investigative agencies than it is of the movie itself.
As far as the movie goes the gore effects, while not always convincing, were at least effective, and at 40 mins the plotless butchering doesn't really have time to grow dull; that said, a single scene of butchery with no real surrounding story isn't particularly entertaining.
As a curiosity, it's certainly of interest to gorehounds, but as this is a scene, not a movie, I can't make it a recommendation.