Turns on a switch whenever you load/save, allowing you to set save/load based common events. You can do stuff like make randomly appearing/stocked treasure chests (FFX), or a combo counter that resets itself if you try to save (the battle chain system of FF12).
This includes a demo, which is rather short, but shows basically how you work this. Normally, you use end events and shut off the switch after calling the event. But I had objects that appear on save/load, so I used another method.