It's a bit of an outdated rule.
"Stomping a grounded opponent.Stomping a grounded opponent is not allowed. Stomps can only be utilized in the standing position against another standing fighter."
There might be a weird situation where they are playing that Weidman game with the hands/palms down on the floor and you can stomp the hands. I think that scenario is quite rare. I don't think it is that effective. Remember those kidney kicks from the guard? They used to be illegal but they should be allowed now. Nobody is using them anymore though.
A pet peeve of mine is these rules that really don't do shit to make anyone safer but rather enforce preventing some PERCEIVED imaginary danger.
For example. As the legend has it, 12-6 elbows are illegal because some stupid asshole on an athletics (likely boxing) commission, saw a guy break cinder blocks with a 12-6 elbow and said:
"We can't allow that, someone will DIE, they will surely fracture their skull!!"
But 11:30 - 5:30 elbows are fine, and a slight arc and everything is honky dory but 12-6= death.
And that stupid ass fucking rule is still a rule and still results in stopping possible fight ending sequences and taking away a natural instinctive strike that should be relatively effective and I do not for a second believe it is any more dangerous than the variety of extremely inpactful legal strikes like front kicks, roundhouse kicks, spinning kicks, axe kick, spinning elbow, ngannou just regular punches, Alex Pereira left hook...
Let them FIGHT.
No biting, fishhooks, trachea strikes, maybe no finger/toe breaking and otherwise have at it.
Let them use all the techniques.
Let guys on their back up kick, that's a no Brainer, why limit the guy on his back from defending himself in any way??
I'm even sort of open to allowing groin strikes and just having fighters wear steel cups (or trying it out and seeing what happens)
I know what I just posted will never happen, but there is a place in between that I hope could be settled on eventually.
Allowing KNEES on the ground with a 1 meter "warning track" so no one's head gets pinned. That's a perfect compromise. It will return danger to North South and to well executed sprawls, thus punishing wrestlers spamming shitty lazy takedowns. More action, more balance of the rules for strikers vs wrestlers, a deterrent to boring tactics, a TOOL for good wrestlers and incentive for guys to pass guard and pass to high side control or North south thus creating more scrambles and chances for sweeps, win win win.