随心而起 means that you still need to have the intention in mind for it to work, so it couldn't be considered 'auto-protect'. That was why Kong Jian got killed - Xie Xun caught him completely off-guard, so he had no time to even think about protecting himself.
As for how strong it was compared to 9 Yang - ZWJ could still talk and laugh while being hit, while KJ had to immediately stop talking at XX's first sneak attack. KJ was also clearly in pain and winded when he took hits on the abdomen, while ZWJ acted like it didn't hurt at all when he was struck (which included hits on the abdomen, as well as on a vital point and from both sides). Also, as soon as XX started using the 7-injury fists, KJ had to repel the force away from himself in order to survive, while ZWJ simply absorbed it (the last few blows from Zong Weixia don't count, since the repulsion was the result of ZWJ sending 9 Yang into his body to heal him, and not his own force being reflected back).
This isn't conclusive though - the Kong Tong elders we saw were clearly much, much weaker than Xie Xun. I do have trouble picturing ZWJ repeating his performance in the same way with XX throwing the punches.
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I don't think 9 yang would have let him just stand there and take their hits, automatic or not. In any case, he did recover fully in an hour and reduced both elders' palm power to less than 20% of their usual strength through the brief contact with his 9 yang.
It does seems that his 9 yang is vulnerable to diversion tactics. In that exchange, he did consciously summon 9 yang to deal with the first pair of palms, but got hit elsewhere by the second while his inner strength was concentrated on dealing with the first (Miejie and Zhou Zhirou basically did the same as well). However, this is not a good example to show whether 9 yang can 'auto-protect' because ZWJ was imposing his conscious will over 9 yang at that point, which presumably would override what it would have done automatically.
ZWJ: Disengage automatic pilot.
9 Yang: Roger, you now have control.
...
BANG!
It is rather hard to find good examples for or against though, since ZWJ got so good that it was basically impossible to take him completely by surprise. How about this though:
不料張無忌心意不動,九陽神功自然護體,變招快極,但饒是如此,背上衣衫也已給劃破了兩條長縫 ,實是險極。
This was from when he was bending over the leader of Hua Shan, and the two leaders of the Kun Lun sect attacked him silently from behind. In this case, it is explicitly stated that he did not even think about doing anything, but the 9 yang protected him anyway. Though in this case, it acted more like an 'automatic warning system' rather than like 'automatic armour'. Just as well though - 9 yang doesn't seem to work well against sharp weapons.
Yea 随心而起 is not quite the same as AUTO PROTECT of 9 Yang.
Only times his auto protect didn't work was when his main power was diverted elsewhere or he switched it off himself, or its the HEAVENLY SWORD.
Same thing can be said about Reverend Kongjian's Diamond Body Invulnerability.Only times his auto protect didn't work was when his main power was diverted elsewhere or he switched it off himself, or its the HEAVENLY SWORD.
忽见柳荫下两个小孩子在哀哀痛哭,瞧模样正是武敦儒、武修文兄弟。郭芙大声叫道:「喂,你们在干甚麽?」武 修文回头见是郭芙,哭道:「我们在哭,你不见麽?」
I attribute this more to the internal energy quantity rather than the internal energy type. It seems that when you have a high abundance of internal energy, you can talk while consuming internal energy; otherwise you have to keep silent. That is to say, if Kongjian had Zhang Wuji's amount of internal energy, he would also be able to talk while using DBI.
Kongjian probably just had a fraction of Zhang Wuji's internal energy.
忽见柳荫下两个小孩子在哀哀痛哭,瞧模样正是武敦儒、武修文兄弟。郭芙大声叫道:「喂,你们在干甚麽?」武 修文回头见是郭芙,哭道:「我们在哭,你不见麽?」
I think TSTL's BGLTWWDZG's (did i spell that right) other worldliness deserves a mention. What other martial art literally reverses aging?
The "auto protect" is only where he had qi. When he had his qi diverted elsewhere, it specifically mentioned that since his qi was not there to protect him, that part of his body was vulnerable.
So insofar as it's 'automatic,' it's also a function of how much qi he has and how much he can afford to distribute it over his body. If he had Xu Zhu or Sweeper Monk level of qi, he would probably be able to engage in great level foes and still have enough left over to autoprotect.
I'm getting the impression that Kong Jian needs to consciously move his energy around in his body to block attacks, whereas Zhang Wuji's 9 Yang automatically moves inner power around for defensive purposes, as long as there is enough available energy to do so e.g. if he is using all his power to fight someone, and someone attacks him from the back, it will not block that attack since there is nothing to block with. But under normal circumstances, if he is hit on the back, the strike will be protected against.
Reverend Rongku prepared himself.
Suddenly, he toss his hands and screamed: "I am not human! I am an animal!"
The crowd startled at such a bizarre beginning to the story.
Kong Dong try to ambush ZWJ when he was talking, but bang 9Yang auto protect deflected it.
I think this is an incorrect assertion. I am pretty sure that it was stated that techniques like DBI relied on 'one breath of qi'. Talking disrupts the focusing or circulation of the qi and this leads to vulnerability in the technique.
Likewise, the masses were astounded when ZWJ was able to talk when he was exerting himself in an inner power contest/struggle. Even the Du monks who have comparable inner power attainment can't do it.
If we go over to the Gu Long universe, there was a character in the Lu Xiaofeng series, Huo Tianqing, who was able to talk when he used his inner power in a contest with Dugu Yihe. But this is contrary to what is deemed possible.