Regarding: “I would think that our default should be to take the classics as prescriptive. Why do you take this phrase as merely descriptive of one master's optional experience?”
I agree that there is plenty of prescriptive language in the taijiquan classics, but what I mean to say by “descriptive” is not that these texts are “merely” descriptive of one person’s experience, but that they evoke the immediate experience of masters. As such, they do not read well as “how to” explanations of movements or stratagems, but are better viewed as expositions of subjective experience, or “what it feels like,” or “how it works.” As for the specific saying, “If the other does not move, I do not move; if the other moves slightly, I move first.”—recall that this appears in the text as a quotation, preceded by, “It is also said. . .” 又曰 So this stands out to me as likely an oral saying that could well have been stated by a master while engaging a student in push hands or other partner training. I’ve encountered a number of translations that read the ji as “I” in this saying, and find it the most convincing and evocative reading. It places the reader in the moment. There is also Chen Weiming’s commentary on the phrase, in which he glosses 己 as 我. He writes:
打手之時, 彼不動則我亦不動. 以静待之. 彼若微動, 其動必有一方向. 我意在彼之先.隨其方向而先動. 則彼必跌出矣.
Barbara Davis translates this comment: “When playing push hands, if my opponent doesn’t move, then I do not move either, and I wait for him by being still. If the opponent moves the tiniest bit, his movements must have a direction, but my intention precedes his. I follow his direction and move first. Then the opponent must be tripped up.” (Barbara Davis, The Taijiquan Classics: An Annotated Translation, p. 141)
As for the much earlier formula about setting out later but arriving earlier, I was just struck by its resonance with the taijiquan phrase. The phrase in the Sunzi, of course, has to do with the movements of troops, and the tactic of “making the enemy’s road long” and to “lure him along it by baiting him with easy gains.” The 後人發, 先人至 (set out later, arrive earlier) phrase was also quoted in the Xunzi chapter on warfare. The similar phrase in the Zhuangzi may be much more applicable to the taijiquan context since it addresses one-on-one sword technique. However, they have in common the notion of enticing the enemy with perceived advantage 利. That resonates as well, I think, with the taijiquan notion of “attracting the other into emptiness.”
Here are the two versions: