To see it in action:

• https://piefed.zip/communities?topic_id=0&feed_id=0&language_id=0&nsfw=all&search=&sort_by=active_weekly+desc

Crust.piefed.social (the Piefed development instance)

• https://crust.piefed.social/communities?topic_id=0&feed_id=0&language_id=0&search=&sort_by=active_weekly+desc

Not all instances implement it yet, I guess it depends how often they pull from the dev branch

Example of search for 'movies'