Hyperventilation techniques can be beneficial, but they should only be practiced by those with good functional breathing habits and under proper guidance to avoid reinforcing bad breathing practices. It is important to understand the context and correct usage of these techniques to maximise their benefits and ensure safety. Bellows breath is a pranayama technique that would have the same negative effects of increasing chronic stress if the person doing it doesn’t already have good functional breathing. I find Wim Hof a little fanatical but the science is backed. Unfortunately everyone just jumps on the band wagon of a quick fix without doing any ground work first.

@Shaun La Touche early stage COPD can see a massive improvement from breathing exercises, late stage may find them more difficult due to already high levels of breathlessness. However retraining nasal breathing and some gentle breath holds (on exhale) and possibly some humming will still have a benefit due to engaging the diaphragm and increasing nitric oxide production allowing any healthy airways to open more for a greater oxygen uptake.