In our lineage it is called Tei Gong.
Relax , back straight , Tei Gong , that is the mantra.
It is a physical and mental thing , if you want to get all esoteric it is thought to facilitate the flow of "Nim Lik" (Thought Force) up the spine and out to the limbs.
It is developed mainly by doing the Siu Nim Tao form for many years.
I don't think you could apply it to Hapkido , you could try it , but I don't believe it would work quite the same way.
A lot of it has to do also with the back being straight and the feet being parallel and focused towards the centerline.
In other words you would have to adopt a Wing Chun stance , and then you wouldn't be doing Hapkido anymore would you?
You might as well just do Wing Chun.
This is why trying to take something from one martial art and trying to bolt it on to another martial art often times does not work because they are just too incompatible.
Once the stance is changed then it's doubtful that anything will work as it should.
Sorry to take so long getting back to you on this.
Is there a good written reference out there that talks about this concept? I believe that Hapkido is one of the more soft internal Korean martial arts though it may not seem that way to watch it. I am not necessarily saying other Hapkidoist feel this way, but I do. I think this concept is worthy of exploration. My particular school borrows some of the interception ideas from JKD so maybe we can integrate it.
Hapkido steals from everybody!