I wanted to add, that you shouldn't focus "too" much on this huiyin info. If you are to focus on the lower dan tien, that is the focus, not the huiyin. Also, the idea, I think Margaret mentioned, quoting Master Lin's description concerning emptiness being a point where you have feeling, but that 'focus' takes away the feeling, or brings you back out of the emptiness.
Focusing on the lower dan tien brings energy there, I think, even up from the huiyin. Closing openings can be a subtle, secondary awareness, barely physically expressed. I am still learning too. Some of the things I am sharing may be work in progress, or universal qigong components. Master Lin's method is very open and "soft"/spiritual....I don't want to put additional "form" into that, inappropriate to his intentions. I am aware that I do something in the area of the huiyin, often, to close it, pull it up... especially , in the past when i practiced systems in low, demanding stances.
I really held it up then... I can't define though, what the mind can do without the body . I was reading some deep Taoist text last night, and when it spoke of , say, focusing on the lower dan tien, and that having an effect on drawing up energy from the huiyin, along with other areas, I felt I needed to add this message. This would be something good to clarify with Master Lin at some point. How much attention, if any, to put there. It is something I currently find myself physically doing to differing degrees, but I also could see affecting that area in non physical/ "mind moving the energy ways", without that focus... How's that for ambiguous and non commital! You can say, or think, a sound.