In one talk of Venerable Maha Bua, he mentions that his practice was always very intense and that Venerable Mun would have to rein him in.  By that he said that he would fast to extreme, not eating for days on end, and also sit in jhana for extended time periods.  Due to the fasting, he said that 10 years into his practice, he had ruined his stomach.  What exactly does that mean? I don't know.  He further commented that Ajahn Mun would chasten him for sitting too long in jhana, at one time saying, "Are you going to die in jhana?"

For the specific account see this pdf, about 3 pages in, where he talks about his practice and how Ahjahn Mun guided him:

"Only we can take care of our citta"

Also, back in the days of Ajahns Mun and Bua, the chewing of betel was very common; nowadays not as much so.  Could Venerable Bua have been chewing betel, or perhaps some other substance?  It is all speculation.

Also, it appears in the video that he has few, if any, teeth.  Could he be chewing something for tooth or gum pain?

 This type of question, in contrast to the content and insights that Venerable Bua is sharing, just seems to be unprofitable.  However, I've tried to give a response.