"All out war" is the strategy of creating a situation in which the players can be fed experience at a constant or variable rate to whatever end the developer desires. An example would using a gauntlet style event to ensure players are appropriately leveled before fighting a boss or boss type event.
An example is World of Warcraft dungeons when a player is using them to level up for another challenge such as a new dungeon or a level split or new skill.
Another very common example is the learning phase in World of Warcraft. The player has their hand held from level 1 to 10 to teach a new player the game, but it goes fast enough that it doesn't become tedious. This pattern can be found in a few other RPG style games as well. This is mostly used with games that are expected to have the parts played through multiple times by a majority of the expected audiences.