This short video encapsulates this in a beautiful way. (https://www.globalonenessproject.org/library/interviews/what-hongi-means)
The haka is also associated with the environment as it was traditionally used to mark a tribe's territory and scare off enemies. During sports games, it sets the mood by hyping up the team and those watching. It is powerful and gets the adrenaline flowing. I imagine this had the same effect on the battlefield.
