While "turning a hook" a card scraper, I got to thinking about how does the hook actually know the proper direction to fold. After some mis-directions from Claude/Sonnet on this. (Don't trust it on the topic), I found an interesting post with micrographs of what's actually going on.

TL;DR: As many of you know, that nice honed corner you put on the card gets mostly obliterated with burnishing, and the hook forms from slumping the burr formed in the prior step. The rod also shears off part of the slump to sharpen it. The phrase "turning a hook" ought to be retired.

Sadly, this article is a digression from the author's scienceofsharp web site, so he doesn't go into what the implications are, like does the texture of the scraper burnishing rod matter, or what shortcuts in the card prep could be made. 

Anyhow, I found the article fascinating, so thought I'd share it.

https://scienceofsharp.com/2019/06/08/what-does-steeling-do-part-2-the-card-scraper/