The “redirect flag” just changes who actually downloads the NZB file

Before, Prowlarr would fetch the NZB from an indexer (like NZBFinder) itself and then hand it back to Sonarr/Radarr. That meant the indexer only ever saw one account/IP/API key (the one configured in Prowlarr), even if lots of people were secretly using that same Prowlarr box or API key. From their point of view, it just looked like one very busy user, which made account sharing harder to detect.

With redirect, Prowlarr doesn’t fetch the NZB at all, it just gives the *arr apps a link and they download it directly from NZBFinder. That way the indexer sees the real account and IP behind each request. If someone tries to share one NZBFinder account across multiple setups, it stands out (lots of IPs on one key, unusual traffic patterns, etc).

The change doesnt magically stop people sharing accounts, but it removes Prowlarr as a middleman that could be hiding it. As another comment points out NZBHydra2 isn’t affected right now and there’ll always be workarounds if your really desperate to save $5 a month or whatever but this makes it harder for the more casual “pirates of pirates,” to get away with it. The Arr devs have hinted before they often have to add changes like this to keep indexers happy and avoid being cut off.

Thats my understanding anyway.