Hi,
First of all, I said it 'may' be considered an advanced breathing technique by some people. It is, just a breathing technique. It stimulates the lower dan tien, and also the front and back channels. There is no mysterious danger wrapped inside. It is a 'basic'. Small universe, or micro orbit meditation, is a 'basic', in Taoist qigong material. The Taoist contributions to qigong, are stellar in promoting longevity and health.
This is a common practice. There is no need for you to be putting my reference to 'some' people out there 'perhaps' calling it an advanced technique, on Master Lin, as his misguidance to you!
I first learned reverse breathing, over 25 years ago. I have never, had any indication from practice, or teaching it, that it has limits or dangers to it. Like anything, there is a right way to do it. Qigong, in general, was not taught 25 years ago. It was somewhat clandenstine, and secretive. It was, in say martial arts, a giving of power to someone. Nothing , to be doled out openly, from the perspective of teachers in power, with power, not willing to 'empower', other people.
You can find whatever you want to hear on the internet. I see old tendencies of holding back information, and calling it dangerous, as part of a heritage of controlling information. All masters, were students, if not still, of their masters. Alot of bad ways get passed on. Not all students have the insight , or power to change the way things are given, to them.
It really doesn't matter what I say! You can find something contrary to it, if you look for it. No one practicing SFQ, in my years in this forum, has had diverse side effects to reverse breathing. All inquiries, have been about the 'how to', or wanting to understand the method better.
There is a good , better, best measure employed in SFQ practice. You can breath how ever you are most comfortable breathing.
love,
gallen