When Snowball is expelled, and Napoleon takes control of the farm, it is announced that the windmill will be built. This surprises the animals, as Napoleon was vehemantly opposed to it. Spinning the truth is a common tactic of propaganda; Squealer takes control of the public discourse with his circular logic and outright lies:
That evening Squealer explained privately... that Napoleon had never in reality been opposed to the windmill... Why, then, asked somebody, had he spoken so strongly against it? Here Squealer looked very sly. That, he said, was Comrade Napoleon's cunning. He had seemed to oppose the windmill, simply as a manoeuvre to get rid of Snowball, who was a dangerous character and a bad influence.
(Orwell, Animal Farm, msxnet.org)
By showing Napoleon as a brilliant tactician instead of as an opportunist, Squealer alters public opinion of Napoleon's brutal regime to one of selfless sacrifice. After all, if the windmill was Napoleon's idea, it was evil of Snowball to take credit for it; additionally, if Snowball had been plotting to overthrow Napoleon and become ruler of the farm, then his actions must be evil, and Napoleon's actions must be good. Without any dissenting opinion, the appeal to fear covers up the outright lies and gives Napoleon carte blanche to do whatever he wants.