I found this on Buteyko: https://www.youtube.com/watch?v=tKaUEVnducI
Interesting, he seems to be saying that we shouldn't take deep breaths instead slow and shallow. So kind of the opposite of many other techniques mentioned here.
The Buteyko Method is based on the idea that over breathing or hyperventilating is bad for you. Contrary to popular belief expelling too much carbon dioxide is actually not good for you due to the Bhor Effect, which basically says that oxygen cant be released from haemoglobin to cells in our body without enough carbon dioxide. By slowing your breath you increase the amount of carbon dioxide in your body , which then allows better oxygenation of your cells due to the Bhor Effect.