Conciseness

The body is dense normative guidance with no padding of concepts Claude already knows — every line states a non-obvious custom rule (「优先使用差异很小但判断不同的例子；做不到单条件对照时，如实说明，不编造伪反例」). Minor over-explanation and some repetition exist (the condition-change idea recurs in 理解标准 4, 公式与原理's 「同一情境改变一个变量」, and Gotchas' simplification note), so it sits at 'efficient; minor instances that could be trimmed' rather than the lean level 5.

Actionability

Concrete, executable directives are pervasive: 「用简单数值跑通一次」「第一句落在核心判断或读者真正关心的疑惑上」「交付前按『会这样跟一个聪明朋友说话吗』检验口语」, plus three named worked examples (似然, 方差公式, 正反馈). However, several standards remain at the criterion level (「能辨认它」「能迁移」) without operational detail, and the Examples section names scenarios in one line each rather than demonstrating them — 'mostly executable guidance with minor gaps', not fully copy-paste-ready level 5.

Workflow Clarity

For an instruction-only single-task skill, the numbered 理解标准 (1–5) functions as an explicit validation checklist and Gotchas supplies pre-delivery self-checks (「检查讲解是否交代新情境下的判断依据、结果推导与条件变化」), with no destructive/batch operations requiring a cap. The organization is parallel standards rather than a sequenced process, and checkpoints are embedded in prose rather than an ordered check flow — 'clear sequence with most checkpoints present; minor validation gaps' rather than the explicit feedback-loop structure of level 5.

Progressive Disclosure

The body is 45 non-blank lines across six clearly headed sections (读者与交付, 理解标准, 公式与原理, 中文表达, 证据与版式, Gotchas, Examples), is explicitly self-contained (「本技能自包含，直接完成讲解」), and needs no external references — all content must load at invocation time, so nothing belongs in a separate file. Under the rubric's simple-skill exception (under 50 lines, well-organized sections, no need for external references), this earns the top score; it is not level 4 because there is no misplaced inline bulk to relocate.

Specificity

The description names its domain (「概念、公式、机制与原理」) and multiple concrete capability outcomes (「讲到能辨认、能推导、能迁移」「接通具体情境、判别依据、运作过程和成立条件」), plus a four-item NOT FOR exclusion list. It is specific but expressed as outcomes rather than the full set of concrete actions, and the deliverable form is left implicit — matching 'lists several specific actions; minor gaps in coverage' rather than the comprehensive level 5.

Completeness

Both parts are explicit: the 'what' is stated up front (「把概念、公式、机制与原理讲到能辨认、能推导、能迁移：用通俗中文接通具体情境、判别依据、运作过程和成立条件」) and the 'when' uses a concrete 'USE WHEN' clause with multiple trigger phrases, additionally reinforced by an explicit 'NOT FOR' list. This matches the anchor 'clearly and explicitly answers both what AND when with concrete trigger phrases'; it is not level 4 because the when-clause is already fully explicit and specific.

Trigger Term Quality

Natural user phrasings are well covered: 「通俗易懂地讲解」「讲透这个概念或公式」「费曼讲解」「ELI5」 plus explicit skill-name invocation and 「希望从零理解一个知识点并能用于新情境」. A few common Chinese variants (e.g. 「深入浅出」「用大白话解释」) are missing, which fits 'good keyword coverage; a few natural terms missing' rather than comprehensive level 5.

Distinctiveness Conflict Risk

The skill occupies a clear niche (plain-language explanation carried to a usability standard: recognition, derivation, transfer) with distinct triggers (「费曼讲解」「ELI5」, explicit name invocation) and an explicit NOT FOR list separating it from polishing/translation (「单纯润色或翻译」), whole-book/paper reading (「整本书或论文解读」), and formal proofs (「只要求形式证明」). Conflict risk is minimal; it is not level 4 because the negative boundaries already handle the closely-related overlaps.