The Rule of Backtracking mainly applies to the game Dead Space, where the player does a lot of backtracking throughout the levels. Basically, if you leave an area where you've killed monsters and then come back, those monsters will have mysteriously respawned. It's handy for players to know, especially on Impossible Mode, that just because you have cleared out an area does not mean that it's totally safe for the rest of the game.

When you eat dirt for the first time and leave your family to build a shack in the woods