Because I responded to the same initial question elsewhere, I'll copy and paste here:
The term 疑情 (yiqing) is really not to be translated as "doubt", although in general modern Mandarin uses the character 疑 (yi) together with 懷 (huai) (懷疑) (huaiyi) to mean "doubt". But this is just the word, and to quote Gombrich, meanings are in sentences and not in words.
It applies to the context of 參禪, which is basically "investigate Chan". In particular, in the Chan practice of 看話 看話頭 參話頭 "watch / investigate the word / word-head". "Word-head" is generally known in English through the Japanese term "koan", but actually "koan" in Chinese "gong'an" means something quite different.
However, I don't think that "investigation" is quite the word, either. It is mainly with regard to the notion of not apprehending the object in the manner that one commonly thinks that it exists, the lack of letting the mind fully take up the objects of cognition. This leads to an absence of grasping on one hand, and also of conceptual proliferation, both about the object in question. Taken to its fullest, it is probably very akin to the notion of the mind which does not take up any object (cf. AN 11:9), totally unsupported mind.
When we look at this phrase in context, we find that it is not at all "doubt" as opposed to "confidence" (sraddha). For example:
近來篤志參禪者少。纔參箇話頭。便被昏散二魔纏縛。不知昏散與疑情正相對治。信心重則疑情必重。疑情重則昏散自無。」(CBETA, T48, no. 2024, p. 1099, c27-p. 1100, a1)
Chan Gate Exhortation: Master Su Angtian of Yangzhou's Teaching to the Assembly:
Recently, those who come to investigate Chan are few. Coarse investigation of the word-head will lead to being bound up by the two Mara's of dullness and scatteredness. Some do not know that the "yiqing" is the exact remedy to dullness and scattering. If one's confidence is strong, then the "yiqing" will also definitely be strong. If the "yiqing" is strong, then dullness and scattering will naturally disappear.
So, from this example, we can see that the "yiqing" is not at all any sort of absence of confidence / faith (asaddha).
Huifeng wrote:It is mainly with regard to the notion of not apprehending the object in the manner that one commonly thinks that it exists, the lack of letting the mind fully take up the objects of cognition. This leads to an absence of grasping on one hand, and also of conceptual proliferation, both about the object in question. Taken to its fullest, it is probably very akin to the notion of the mind which does not take up any object (cf. AN 11:9), totally unsupported mind.
Thank you for this - I haven't seen it put this way before. If I may ask - a totally unsupported mind sounds like quite a lofty aim, doesn't it? In meditating on a huatou (word-head) the Great Doubt stops the meditator from settling on a mental object, is that what you are saying? But I guess the mind is still supported (in the sense of "abiding") by a notion of a self and all the consequent reification?
My phrasing above was mainly in the light of the fact that this is in a Theravada Forum, and it appeared that there was a fair amount of confusion, mainly about reading 疑情 yiqing as "doubt" which was mistakenly considered the opposite of 信 xin "faith / confidence". If I was just responding to a Son (Zen / Chan) practitioner such as yourself, I might have phrased it differently.
Thus, the "unsupported mind" is a term straight from the Pali Canon. In Chan, I'd rather saying "non-abiding mind" 無住心, which is mentioned by the Sixth Patriarch Huineng. Or, for a Son practitioner, (not that I know much about Son per se), I'm thinking that the now common English term "don't know" may be in order.
Anyway, like a lot of terms, we may use that term as both the practice, but also the result. (But personally I don't like to use "practice is realization" in the sense that Soto does.) Rather, like "emptiness" (even in the Pali canon), we can use this term to indicate a practice - the emptiness samadhi, the emptiness abiding, etc. - and also the result, the empty mind (empty of afflictions / conceptual proliferation). So, only part is a "lofty aim".
At first, one really needs to settle / abide the mind with some sort of samatha, calm it down. Then, pull up the object in question, and raise the word-head. eg. classic Chan would be to use recitation of Amitabha until one has some good Amitabha samadhi going on, and then ask - "Who is reciting Amitabha?" These "who" word-heads are great, because they then turn the subject ("me" / "I" and "what pertains to I") into the object of the "yiqing".
Most people would just say "I", "I recite Amitabha". But, then one begins to 參 (can) "investigate" this, deeper and deeper. For those who haven't much theoretical training in Buddhism, especially the notion of "not self", they may ask: "So, what is this I?" "Is this consciousness I?" "But this consciousness changes..." and so on. For those with the background, then the simple question "Who recites Amitabha?" Will be enough to raise the strong "yiqing". Rather than identifying as "I recite", "the name Amitabha is recited", and "this is recitation", one "empties the three aspects" and cuts off the basis of "self".
At first, this will be a kind of reified "not self". ie. rather than the usually conceptually proliferated idea of "me" and "mine", one instead overlays a different conceptual antidote of "not self". This is still concept versus concept, removing the false with the true. But, this conceptual "not self" is still merely a concept, one is still abiding in the antidote, still abiding in the word-head, so to speak, one needs to go deeper.
While it is still conceptualized, it isn't really "yiqing", but just rational thought. Only when it cuts out this inner verbalization, inner talk, does it fully develop into the yiqing. One may merely raise the word-head just enough to sustain this. For some, the raising of the word-head once, may be enough to sustain the yiqing for an hour, or hours, or even days. This takes some serious gongfu, however. For most beginners, maybe they'll have to raise it up every few minutes or so at least. haha! Just don't babble away and recite it like a mantra or something - totally, totally different kettle of fish!
Only when one removes the actual basis of "self" and "mine", will the conceptual proliferation end. It may take a lot of time. This is the first break through. Examples could be such as Master Hsu Yun, who in his six year "three steps and one prostration" pilgrimage, entered into deep samadhi while walking and on pilgrimage. He maintained the investigation and yiqing for a long, long time before he had his realization at Gaomin si in Yangzhou.