Are you talking about the system where you put buckets above a rain gutter which has a float valve to set the level? E.g.
https://www.youtube.com/watch?v=NyOIRVjatdg
That's actually quite clever! Though I'm not sure how they avoid the gutters from turning into algae blooming, mosquito-breeding cesspools. :p
Edit: Oh I guess you can just cover the gutter in plastic and ensure there is enough water throughput to stay fairly fresh: https://permaculturefoodforest.wordpress.com/2016/04/14/rain...
