This illness [of having something to obtain] is not just that of particularly able and virtuous members of the scholar-official class, for Chan monks of long practice also contract it. Most are unwilling to "take a step back" [i.e., to forego running after intellectual knowledge, and instead to search out where intellectual knowledge arises from] and do gongfu in terms of saving on expenditure of energy. They just, with their clever intelligence, calculate and engage in mental reflection, rushing around seeking on the outside. Even if they happen to experience --outside of clever intelligence, mental reflection, and calculation--a good teacher's demonstration of the "fodder for the original-allotment" [i.e., the stick, the shout, verbal teachings, and so forth], most of them, in the face-to-face encounter with the teacher, end up mistakenly thinking that the ancient worthies of old did have a "real" dharma to give people, such as Zhaozhou's put it down and Yunmen's Mt. Sumeru.
Yantou said: "Expelling things is high, and pursuing things is low." He also said: "The essential directive of Chan is that you must be on familiar terms with the phrase [i.e., the huatou]. What is phrase? When you are thinking of nothing [but the key word/phrase] at all, that's called correct phrase [i.e., correct huatou practice]. It's also called residing at the pinnacle. It's also called abiding. It's also called clear. It's also called wide-awake. It's also called the in-that-way moment.
By means of that in-that-way moment, one uniformly annihilates all affirmation/negation. But as soon as [you become fixated upon] in that way, [no longer is it thinking of nothing at all, and so] it's immediately not in that way. Any affirmation of the phrase or negation of the phrase is to be shaved off. [The perfection of prajna] is like a ball of fire--if you touch it, you'll be burned. There is no way to approach.
My addition: In the Ames Mahasangha, we usually use the huatou "What is this?" Do not let yourself answer (fall into affirmation or negation, in Dahui's terms). If you must answer, you may answer "Just this" or "Don't know." Return to this question every few minutes, if you can. You will begin to see how it helps to cut through delusion and reduce our distress.
(Excerpt from The Letters of Chan Master Duhui Pujue, by Jeffrey Broughton, pp. 62-64)