that is circular breathing, it is so they can produce uninterrupted sound for longer. It is hard and I've tried but can't do it. I had band leaders and friends who could do it. It is definitely not something that you could apply to physical work, you don't get enough volume in or out.

i read a thing this week that breathing OUT is more important than breathing in because the CO2 is more detrimental to your performance than lack any lack of O2.

*insert Hitler spoof video with Conversational pace*