Active has a 48-hour cut-off, and the ranking function it uses seems to encourage the same few posts to stay at the top for 48 hours. It's basically the same ranking as "Hot", but using the timestamp of the last comment instead of the time of posting to decay its ranking over time.

This means any comment activity whatsoever on a popular thread bumps it back up the rankings significantly, and I suspect leads to a kind of snowballing effect that keeps posts higher up. Ideally, it would use some metric based on user interactions over a time period to calculate a score of activity rather than solely the latest comment. In effect, it seems to act more like a "top from last 48 hours". (Although I would add I'm a newbie to Lemmy, so might not yet have an accurate picture of its behaviour).

Lemmy seemed to get much livelier for me when I switched my default to Hot, but I wish there was a way to disable the auto-updates (I'd rather see new items only on browser refresh). Active sort feels pretty stale to me.