When a big backlink disappears, rankings can fall in proportion to how much of the page’s authority that link was carrying and how hard it is to replace, and the drop usually shows only after a re-crawl lag. A link that was doing real load-bearing work, supplying a meaningful share of the signal holding a position, leaves a real gap when it goes. The size of the effect scales with the link’s uniqueness, and the whole thing is observed cause and effect rather than a precise published rule, so the complacent “one lost link won’t matter” is wrong in the cases that count.
The scaling is the heart of it. Not every link carries the same load. A page propped up substantially by one strong, relevant, hard-to-replicate link is exposed if that link vanishes, because nothing else is doing that work. A page whose authority is spread across many comparable links barely notices any single one leaving, because the others cover for it. So the question is never just “did a link disappear,” it is “how much was that link carrying and could anything replace it,” and the more unique and load-bearing it was, the larger the fall.
The timing comes from how the loss is registered. The engine does not see the link vanish the instant it is removed, it learns of it when it next crawls the page that used to host the link and finds it gone, then reprocesses the change. That means the ranking effect arrives with a lag, often days to weeks depending on how frequently the linking page is crawled, so a position can look stable for a while after the link is already gone. Putting an exact day on it would be inventing precision the crawl cycle does not give you.
So the mechanism is straightforward: lose a link, lose the authority it carried, in proportion to how irreplaceable it was, on a delay set by re-crawling. A unique strong link is a single point of failure, and pretending its loss is harmless ignores exactly the link most worth protecting.
Map which inbound links are genuinely load-bearing for your important pages, then monitor those specifically and have a plan to replace or earn an equivalent if one disappears, rather than assuming any single lost link is too small to matter.