Quid significat "caeca riveta saltatrix"?

Caput saliens clavi caeci: quia clavus caput clavorum non involvit, caput saliens clavi faciet.

https://www.yukerivet.com/news/the-core-of-the-blind-rivet-is-not-complete-pulled-out-what-is-the-reason-for-the-fracture/