It finds the first comment written after your last visit, places the new-comments divider right before it, and the divider slides down slightly from above and settles in place

Read comment 1 — already read on a previous visit.
Read comment 2 — saw this one last time too.
Read comment 3 — this is where the last visit ended.
New comment 1 — posted after your last visit.
New comment 2 — below the divider it's all new.