A torrent left the cache: relaunch it
When space runs low, unused torrents leave the cache. Your Library keeps them, and a pass relaunches them in one click.
The cache is large but not infinite. When it fills up, the torrents nobody has used for the longest time leave it first. Using a torrent means downloading or streaming from it, or adding it again.
Protected for 30 days
While your pass is active, a torrent in your Library that you used in the last 30 days is never removed to make room.
Relaunch
A torrent that left the cache stays in your Library as Removed from cache. With a pass, press Relaunch: our servers fetch it again, as long as it still has peers. Until it's ready, it counts as one of your 20 downloads at once.
Relaunching needs a pass. Either way, don't use Ninjadebrid as the only copy of anything: download what you want to keep.