Se cayó... acción involuntaria. Se dejó caer..... decidió no resistir, decidió perder la decisión de estar de pie....pero "don't give up". Peter Gabriel diría:no te dejes caer de rodillas 😉

Hi Duncan, “Se dejó caer de rodillas” is a bit poetic. He gave up and let himself fall down on his knees. He stumbled and fell down on his knees. That’s the difference. IHIH LEO

Yes, you just got it. La diferencia que existe es la que presentas en inglés. En la forja “dejarse caer” está presente la voluntad de caer; naturalmente, en “caer” no hay intención de hacerlo, como cuando nos tropezamos torpemente.

I just read something that seems salient here. The Spanish book I'm reading explains that dejar + inf. is used to express "to allow to, to let". One example (which led me to revisit your post) was "Dejé caer mi libro." ("I dropped my book.") This would seem to be an analogous situation to your post where you could say "Dejé caer mi libro." or "Se cayó mi libro." From my very non-expert point of view, it seems to be an active vs. passive pattern. "Dejé caer mi libro" = "I dropped my book" "Se cayó mi libro" = "My book fell" Therefore, this seems to validate what you said: "Se dejó caer de rodillas" = "He dropped to his knees" (active - he intentionally allowed himself to drop) "Se cayó de rodillas" = "He fell to his knees" (passive - he involuntarily fell) Maybe I haven't added anything substantive to what's already been explained by native speakers, but I thought it was interesting and wanted to share :)