The root is heightened aggression/anger levels i would guess. Same thing with boxers and power lifters slapping the shit out of themselves before a fight/competition. Anything that gets your blood pumping and adrenaline levels up.

I would hope the 'neutral word' was said at the same volume/rate/tone. Otherwise, yeah, it would definitely be a factor. Just like we can make ourselves angry with our thoughts, there must be a large difference in adrenaline released depending on how you emote.

Made me think of tennis players. The sounds may really increase their power.

It's called a kiai...

I think they used "neutral word" to determine the baseline

This would be a placebo, no?