It might help to think of it as going beyond zero interest rates to the point of intentionally punishing traditional savings to try to force consumer spending (if you hold onto it, we'll devalue it), entice borrowing (we'll essentially pay you to buy a house [1]), etc. There are various approaches to pushing rates below zero. Japan and the ECB have done a lot of experimenting, the US will probably take some notes from them when it comes time to push US rates below zero persistently.

[1] https://www.wsj.com/articles/the-upside-down-world-of-negati...