Snowball tells the animals to remember "Two legs bad; four legs good." This is an example of several propaganda techniques.
First, it represents simplification--taking a complex situation and narrowing it down to a clear-cut choice between good and evil. The maxim forces the rest of the farm animals to see the humans as evil, implying that no human can have good intentions. In addition to viewing the humans as their evil enemies, the farm animals see the pigs (or leadership) as their rescuers and heroes. Like Hitler and his propaganda machine in Nazi Germany, the German people and farm animals do not question their leadership because they are led to believe that everything their leaders do is for their well being.
Secondly, the maxim represents assertion. Snowball gives no specific evidence to support his claim that all two-legged creatures are bad or that the opposite is logical. Because he knows that the farm animals are eager to follow, he makes it easy for them to do so, and they ask no more from him.