Understand Skill

Reads dense content and hands back understanding, not just compression. The output has four layers: a TLDR you can repeat to a colleague in the hallway, a so-what naming what the reader should do or believe differently, the key points that must survive the summary, and an analogy that gives the concept a shape -- with an honest note on where the analogy stops being true.

The difference between this and a generic summary: a generic summary preserves the vibe and loses the substance. This skill does the opposite. The numbers, the caveats, the trade-offs, and the one surprising claim survive; the padding, the tone, and the structure of the original do not.

What Must Survive

When compressing, these carry the meaning and must never be dropped or blurred:

• Numbers: latencies, percentages, scale figures, benchmark results, costs, dataset sizes. "Significantly faster" is a loss; "3.2x faster on reads, 11% slower on writes" is the point.
• Caveats and conditions: the "only when", "except if", "assuming that" clauses. A claim stripped of its condition is a different, usually wrong, claim.
• Trade-offs: what was given up to get what was gained. If the original paid a price, the summary names the price.
• The mechanism: not just what happens but the one-sentence why. "The cache made it faster" loses the mechanism; "moving the lookup off the request path made it faster" keeps it.
• The surprising claim: the one thing in the content a knowledgeable reader would not have guessed. This is usually the reason the content exists; find it and lead with it.
• Named things: systems, algorithms, papers, tools the content builds on. These are the reader's hooks for going deeper.

What does not survive: the author's windup, the recap paragraphs, the third example of the same point, the section structure, and any sentence that exists for tone rather than information.