In chapter 3 of the novel, Snowball declares that the Seven Commandments, which the lower animals are having so much trouble memorizing, might be reduced to one easy-to-remember motto: "Four legs good, two legs bad." This simple motto contains the essential point of Animalism, which is that all animals are comrades and all humans are enemies. If the animals remember this simple motto, then they will always be wary of humans, and humans are the biggest threat to their new way of life.
However, years later, in chapter 10, the sheep start bleating a new version of the motto: "Four legs good, two legs BETTER!" The bleating of this new motto coincides with the pigs being able to walk on two legs. When the lower animals see the pigs walking on their hind legs, they are shocked and speechless. At this very moment, to stop the lower animals questioning what they are seeing, the pigs instruct the sheep to repeatedly bleat this new, updated motto. Any troubling questions being pondered by the lower animals are now drowned out and replaced by a clear reminder that the pigs, who now walk on two legs, are their superiors.
The modification of Snowball's original motto marks the point at which the pigs' metamorphosis into humans becomes all but complete. Apart from their outward physical appearance, they are now essentially indistinguishable from the human oppressors they once fought against.