Kayin's secret save is a function involving saving, probably made as a test and forgotten about. The following are the directions as posted by its founder:
"If you go in the saves folder, you will see 8 saves, 4 save files and 4 paint files. Where it says "save0" rename this "save4" then rename "save1" as "save0". now rename what is "save4" as "save1". you will be able to acces save0 by going and loading game1."
After the following those directions, the end result will appear as the exact same game, but The Kid starts in the Game Over Room. The difficulty is set on hard, and he'll spawn in the Game Over Room until he gets a Save Point.