it has to do with pagination of database results.

traditionally people used links like ?start=10, however if new items were added in between the time you loaded the page and the time you clicked the link you would see them on the second page if the result set were sorted by newest first DESC

so then ppl used things like ?afterID=10thID, however that doesnt really work with a site with a voting sort, that item will move around from first place to last place on the page and you could get a whole new batch.

hn uses some system, possibley the afterID system, but it knows that after N seconds or N changse to the table the list is no longer valid.