What is the meaning of "high isnad" and "had it fallen, it would have descended" in the context of al-Mustakhraj's narration of hadith through the chain of al-Bukhari?

The term "elevation" ( uluw ) refers to a smaller number of narrators in an isnad (chain of transmission), while "descent" ( nuzul ) refers to a larger number. Mustakhrajat works, such as Abu Nu'aym's Mustakhraj on Sahih Muslim, achieve uluw of the isnad due to their fewer narrators compared to narrating the hadith through the original author.