The 5 second rule was a rule which was implemented to allow players to only return cheese after 5 seconds of gameplay has passed. The rule was added to prevent hackers from returning the cheese first, and also to prevent a glitch where the player starts with the cheese on the hole.
The 5 second rule used to be a problem on some official and unofficial maps. It turns a race into a waiting game. Maps like map 26 are possible to be beaten in under 3 seconds, but with the 5 second rule, mice who return the cheese too fast were instead killed.
With update V1.115 it was possible to bypass the 5 second rule on levels with a shaman. If the shaman died as soon as the round started, the timer went to :20 seconds left like it normally does when the Shaman dies (Unless if they activated the "Ambulance" skill), but the map clear time did not go to 100+ seconds. With the removal of the five second rule from gameplay, this timing is still in the game.
When the rule was first put into place, it was for 8 seconds. It was changed to 5 seconds after the developers received complaints that the cheese could be returned in just 5 seconds on certain maps.
With the fix of the cheese carryover glitch, this has been removed.
- Five-second rule on Wikipedia, a popular food myth.