I don't generally use this since I tend to do a lot of other breath type work during an attunement, but once in a while I do when it feels like the best thing to do in the moment. And sometimes I only use it for part of an attunement instead of throughout the entire attunement like others do.
The Hui Yin point is Ren 1, the first point of the Conception Vessel and the meeting place of three different meridians.
If you know how to do kegel exercises you can adapt to this easily. If you don't know how to do kegel exercises I suggest learning how to do those first as it will make using this technique more doable.
However, feel free to ignore it. Usui never used this technique, so there's no need to worry about it if this is confusing to you.
Reminder, this is for an attunement. Holding yoru breath for an hour session isn't that great. Also, this is something you are doing to YOUR body. The person you are attuning is not doing any of this, and of course, you aren't going to touch a person's private parts.
Feel free to ask me any questions regarding this technique in the comments.