Today I found out that if I make the highest possible pitch 'e' sound to our #2 (non-dominant) rooster when he is about to kick me in the shin when he's frustrated (after he had a run-in with the dominant rooster), he stops his aggressive behavior, turns away, starts picking the ground, etc. He looses his aggressive intent when he hears this. I had the opportunity to confirm this 3x today.
I think the sound might also alarm the others (dominant rooster and chickens) when they hear it, so that they move towards shelter but I didn't have the chance to concentrate on them when I deterred #2 rooster. It might be that the sound reminds him/them of a predator bird but if so, why would the #2 rooster not run for shelter? Maybe because he knows I'm usually his/their protector? On the other hand, the dominant rooster sometimes make a high pitch sound, so maybe that's the explanation. In any case, this seems to work to avoid a bruised shin or knee.
I'll see if my 'e sound' is able to avoid (or perhaps even stop) a fight between the 2 boys.