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The concept it similar to hua jin (mysterious power) from Xingyiquan, or our own I Liq Chuan concept of "mystery hand", the point at which your skill becomes so precise that the effect you manage to achieve on your opponent seems "mysterious".
Unfortunately, in modern times, there's been a lot of non-sense that's been built up around the ideas of what lingkong jin is, mostly being propagated by Westerners. You can find a bunch of ridiculous demos on YouTube of supposed lingkong jin, which are something between circus tricks and brainwashed compliance by the demo dummy.
I found the post so interesting because it tracks with my understanding: it's about preciseness and the role of the attention and is easily neutralized of the opponent has the attention to keep up.
I have translated the bit of info about Lingkong Jin that Chen Yanlin got first hand from the early Yang family and published in 1945. FYI — Lingkong Jin / (Going thru) Void Strength Lingkong Jin is boundless and marvelous, almost mysterious, hard to believe if not witnessed with one’s own eyes it is something that actually affects the mind.
Highly skilled people who issue this ‘Jin’ only need to utter a sound “ha”, for the opponent to immediately lift his feet off the ground and retreat. This is probably due to the fact that the mind of the receiver is attracted [influenced] by the issuer, so the receiver cannot resist.
In this respect, if the receiver has already learned skills (Jin) like sticking, adhering etc he can feel and foresee this ‘Jin’ as soon as the issuer starts “ha”, and by [simply] retreating the issuer will get no results. With regard to this ‘Jin’, practitioners should not explore in depth [waste too much time over it] but simply take it as a game.
It is said that (Yang) Jianhou and son (Yang Shaohou) could attract [influence] a candle flame within the range of about one foot distance, blocking the flame with one hand then extinguishing it. This is one [another] kind of Lingkong Jin. It is said that this kind of kung fu today has already disappeared.
— Written by Chen Yanlin in 1945 as reported by the Yang family
淩空勁 淩空勁奧妙無窮，近於神秘，非親眼目睹難以置信，實乃一種精神上的作用。藝高者發此勁時，僅須口中一哈，對方即雙足離地而後退，大概是因爲被發者精神已被發者所吸引，無法抵抗。 對此被發者如果已先知沾、粘等勁，在對方一哈之後，即由感覺而後退，發者也不會産生效果。對於此勁，學習者可不必深求，僅做遊戲看待即可。 相傳建侯、少侯父子，能吸引燭火近尺，一手隔之，火光遂熄，這是淩空勁中的一種。據悉此種功夫今天已經失傳。