Is this manual intervention, or is HN testing out some new heuristic?
This is manual intervention. dang mentioned that they manually boost older article that slipped through the cracks.
I don't know the exact mechanics, however.
Think about it logically. Not every person is going to see every single new submission. It takes time for the submission, if it is any good, to gain momentum and rise to the level that more people will see it. The more people that see it, the more votes it will receive.
Unless your submission is really popular or noteworthy for some reason, 8 hours seems fairly typical for an average submission.
I was thinking out loud because dang has been speaking of HN testing out a new feature to resurrect posts from hellbanned users (and because HN has been sending out emails asking people to repost stories), and assumed some other mechanism is also being tried out.
How this currently works is that when we notice a substantive overlooked story, we add it to a pool. Every now and then software picks one from the pool and randomly places it near the bottom of the front page. It does that by reducing the time decay and/or adding a small number of points. If too much time has gone by for a small change to suffice, the software doesn't try, and instead we sometimes email an invitation to repost the story.
This system would never add 15 points, nor place a story as high as the middle of the front page, so if that's what you saw it means the community noticed the post at the bottom and took it from there—which is the intent of doing this in the first place. We don't much care which stories get taken up this way vs. which don't. The idea is to do the minimum necessary to give high-quality stories a second chance and mollify the cruelty of /newest.