They say that the supernatural power incarnates into the pronounced words and make it come true.
This is kotodama, the soul of words.
It is why people avoid certain words or certain subject at for exemple, wedding party. People don't use shi to say 4, but use yon. (Both two words are to say 4. Shi is same phoneme as death. People avoid to pronounce and utilise another way of counting yon.)
This is what I learned as Shinto culture.
As a singer, I know how to read people's mind by voice. This is all people actually do. The voice is all about inspiring-expiring. When you are nervous, you breath by your shoulder. We singers say breath is up (or something like this). Then your vocal cord is strained, your voice becomes metallic.
The person who has confidence on himself, is relaxed, not tension to his vocal chord.
Thinking about this, the Shinto Idea of kotodama may come from something very simple.
When you deeply relaxed and pronounce something, you may hypnotize yourself. Then it had better to say something positive. Not the bad things but good things. If you just say "I try it", you just realise yourself being trying. It had better to say "I do it" to realise you are doing it in the futur.
All these are totally inadequate for my dissertation. But this is what I heard during my fieldwork from people there. If the ethnologic work is to "encode" the words from the field to the words of observer's culture, I also have to write this in somewhere as well. Just having knowledge doesn't help anything. Once you are capable to act as the lessons you got, then we can finally make it useful our experiences. I have to transmit as well that In the field, I got such precious people with great wisdoms.