The idea was to be able to quickly select things to be made visible on both monitors.
The two monitors are named primary and secondary.
To select something to be visible on primary you would press the combination (for example alt+f1) and the selected desktop would be placed on primary. Whatever was on primary would be pushed to secondary.
To select both primary and secondary you would first press what you want on secondary and then what you want on primary.
The idea is that people typically have what I call "scenes". Scene is particular arrangement of windows on both monitors.
This means that you can quickly learn your "scenes" and get them to be visible very quickly, in a fraction of a second, without having to hard-code the scene in your config file.
Sorry if this is really well known I'm new to MS Windows as a user.
I use dual monitor, but have two virtual desktops giving me 4 "screens" to layout windows in. I keep the same window in the same place, but often have to use 3 docs at once and so would love a 3rd screen. Then I'd have reference material on 1), doc I'm responding to on 2), doc I'm writing in also on 2), citations on 3).
On Mac, Quicksilver with Triggers: https://qsapp.com/