ah.... honestly this part is so insignificant that I recommend you not to spend your time  so much.

However, since you're curious, the definition isn't comprehensive in the first place (because like I told you, this part is not important so I didn't really put much attention into explaining the pitch theory in my textbook).

The alternation of firing itself only occurs for intermediate pitch from 500~5000HZ FYI. That is why I just said that it explains intermediate pitch.