I only did the basic free thing, and found it an interesting experience that calmed me down a lot. Personally I've moved on to other breathwork systems that I think accomplish the same things, but I like better- I practive several of Wim Hof's breathwork methods, as well as the breathwork training that freedivers use.

All of them involve intentionally and temporarily invoking hypercapnia (high CO2) and hypoxemia through slower breathing and/or breath holds.