Five second rule

The "five second rule" was a former mechanic which affected mana regeneration. It prevented a character's mana from regenerating 5 seconds after casting a spell. This was an important concern for mana-using classes, especially healers. It was removed in patch 4.0.1.

Because the activation of the five second rule was dependent on mana expenditure, mana regeneration would continue ticking if a character cast a "free" spell by the means of clearcasting. The same applied to any spell (i.e. Nature's Swiftness) which cost 0 mana. This could be useful when trying to regenerate mana, but still being able cast one free spell without interrupting mana regeneration (in case of an emergency for instance.)

Prior to the 1.4.0 patch, you also did not regenerate mana during the casting of the spell. In-game textual references to the five second rule still read "during casting" where they actually should read "after casting".

Mana per 5-sec (MP5) item bonuses did not fall under this rule and were applied equally regardless of recent spellcasts.