Heavy metal has always been about excess.
The proto-metal bands, like Led Zeppelin, were heroically larger than life. The drummer drank himself to death, they inserted fish into the birth canals of groupies, and had their own custom-painted airplane to take them where they needed to go.
In fact, in pursuit of more, more, more - we had gimmicks like the mighty Thor, who basically was signed as a Siegfried and Roy performance piece as opposed to blue-collar denim and leather. Motley Crue painted themselves up and wore spandex and studs. It got so ridiculous that a movie, This is Spinal Tap, parodied the excess and the pursuit of yet more. ("This amp goes to 11").
Metal itself responded with: "Hold my beer".
Some bands played louder. Motorhead were legendary for being punishingly loud. Manowar were clocked by the Guinness Book of World Records as having the loudest performace ever, outside of the fictitous band Disaster Area from The Hitch Hikers Guide To The Galaxy.
Some bands went for more theatrics: How do you add to the fire breathing of Kiss or the glam of Motley Crue? You invest heavily in B-movie horror props. W.A.S.P. showed up with the kind of set that made Alice Cooper shake his head decrying copycats, but it was really GWAR that took it to the next level, showing up on stage in actual costumes portraying an alien race of Gods, while throwing people into (pretend) meat grinders and hosing the audience down with stage blood.
Some detuned their guitars or added more strings, in an attempt to get lower, darker voices. Some went entirely into sludge metal or Doom metal playing little more than very detuned bass drones.
The vocals? From high pitched singing, to shouted lyrics, to gutturally growled stylings.
At some point, the whole thing needed to have someone just push all the settings to 11 and see what happens, and "slam metal" seems to have gone that route.
To give you an idea of the aesthetic here: the drums are required to be double bass played extremely fast, punctuated by blast beats. And whereas Keith Moon liked to crash ride - you will find them "riding" a china cymbal a lot. For those uninitiated, it means the person alternates between doing fast drum rolls with his feet to simply whaling on the bass and snare in a very rapid one-two march beat - that is, until things slow right down so you can just get how heavy they're going, before speeding right up again. This gets you the best of both worlds - both the opportunity to bang your head slowly and with vigor, and then to jump into a crowd covered with pointy metal and engage in a mosh pit with them.
And herein lies the genius. Simply doing something further and harder just eventually makes you really dislike what it is: so what the progenitors of this sound have done is to pick multiple things - slow brooding moments and speedy thrashing: wall of noise chords and spidery single note runs on the guitar, etc and switch between one and another so that you can appreciate the extremeties of both.
The guitars consist of palm muted, "chugging" chords that punctuate space with some technical virtuosity thrown in with an ascending guitar solo line to break up the rhythm - that way you know this isn't some kind of punk band - but also so that you aren't battered with one kind of sound too long.
One thing that remains constant however is the vocals, which are incomprehensible. Imagine if Cookie Monster had just gotten out of rehab after a decade long bender of Jack Daniels and Marlboro Red cigarettes, and had decided to growl in the lowest possible range of his voice - while simultaneously producing a high pitch overtone similar to Tuvan throat singing. And deciding the only things he's going to say are OORGH and OO-EE OOOO-EEEEE, followed by OOHEEEEEEEEEEEEEEE towards the end. There are actual syllables in there if you really really listen hard enough, but the only way anyone has ever understood the lyrics to any of these bands is to read a lyric sheet.
And the names? Well, a real one is "Abominable Putridity". Let's see, there's also "Vulvodynia", which means pain in the vulva. As an aside, these guys seem to have a problem with the human vulva, especially their obvious lack of access to enjoying any - hence other bands like "Vulvectomy", and/or many a disturbing reference to damage to the genitals, especially female ones.
There's also "Extermination Dismemberment". A while back I joked with a metal fan who is a friend that eventually metal will just turn to grade school schoolchildren to come up with band names and song titles, who would gleefully come back with names like "IMPALED NUN VAGINA BLOOD SPLATTER" or something like that. And we're really not that far off. Vulvodynia has a song named "Castration Mutilation". "Extermination Dismemberment" has a song called "Disembowled Engorgement". And we have "Cemetery Rapist" with "Plundering Into Vaginal Demolition". "Necromorphic Irruption" (sic) has a song called "Transmutative Invader Fetus".
Now in fairness, not all of these are necessarily slam metal: some are deathgrind, porngrind or whatever custom moniker a band decides to use to disambiguate it from every other band that sounds like them.
As for merchandising - there are two rules. One, the cover must be a well-executed painting of some kind of Lovecraftian horror or Gorn, which either skeletal demons or amorphous horror shapes with claws and fangs, or leather aproned serial killers with huge muscles and masks. They are either bursting out of the ground somewhere, chasing someone through a hospital or research facility, or in some kind of subterranean torture dungeon. Double points if a pretty young woman is being held, stabbed, or there's some idea that there's some kind of rot or damage to her genitals. As for the logo, it must be something that Strong Bad out of Homestar Runner desgined, with so many "spinities" and spiky extensions that the actual letters that the glyphs represent completely lose their meaning. "Craniotomy"'s logo is almost legible. But try working out what "Begging for Incest"'s logo even remotely says, even if you see a reasonably reproduced version like on their album "Orgasmic Self-Mutilation".
And right there, having written all this - it seems like a gag out of Berke Breathed's Bloom County which mocked Deathtongue's ridiculously over the top song titles.
Some related genres try to take the general aesthetic and do something progressive with it, however. Cattle Decapitation, while being a -grind variant and therefore having better vocal stylings, uses the horror aspect to promote veganism opting instead of tired power against women tropes to simply depict to humans what is routinely done to animals, for example, on "Humanure" a cow defecating human remains. Suffice it to say that only those with the very strongest of stomachs try and find their video for "Forced Gender Reassignment". Somehow this sort of thing has proved to be more offensive than some kind of over-the-top Satanic horror projection, leading to very heavy censorship of their albums. But for the most part people seem to be content to leave the very fringe of the very fringe alone, content to let anyone overly passionate about this sort of thing to their own self-punishment.