TL;DR It's like live-reload applied to a single module (in terms of JS modules), you change a file (ReactJS component/ redux action or reducer), kick save - the module reloads almost instantly and you see the changes in browser WITHOUT RELOADING THE WHOLE APP. It might be hard to set up.

I do that in a config file, the structure of the app is clear and you don't need to browse multiple files to understand the data logic. If there're very few reducers, thought, i won't bother...