The mantras uttered are supposed to convey the desired grants from the devata. The author says that it should be uttered in some particular way, no body knows that correct way. If you succeed, you presume that you have said the right way. If you dont succeed, then you presume that the mantras were not expressed correctly or with the righ pronounciation etc.
The mantras have a particular meaning. It is a clear expression. Unless it is uttered with an understanding, you cannot express the desire. The devata, generally resides in the mind of the person performing the yagna. Only when the mantra is understood and expressed with the clear intention, the desired results will become plain to the mind and it will start working for it.