I watched a martial arts programme once where these guys learned to shout really loud for something.
Haha, good explanation...
I think it was to either smash glass or maybe push their opponents. I've made it sound horrendous but the actual footage was impressive.
As for learning a thuum, someone crazy may have tried in the hope it works. Like someone trying to go into 'devil trigger' and expecting that to work.
Like a Kiai shout (that is probably spelled wrong). It is believed in some martial arts that the shout can increase the power of the blow. I dunno if it has any basis in reality, but there may be some sort of psychological effect that shouting while delivering a blow imparts. I know as a kid that I was encouraged to do it when I was taking my martial arts classes, but back then I just thought it was an excuse to shout. Who knows?
To clarify, I was not encouraged to just shout random syllables at people for magic to happen. I was encouraged to shout as I threw a punch or kick, as a way to add emphasis to the movement.