Napoleon uses the word 'voluntary', yet the fact that it will result in reduced rations means that there is no choice for the animals - he is contradicting himself. This work was strictly voluntary, but any animal that absented himself from it would have his rations reduced by half. Napoleon announced that there would be work on Sunday afternoons as well.
Napoleon often contradicts himself or 'Animalism' in the messages he puts out through Squealer. The sense of threat in the way Napoleon speaks 'very quietly', is sinister and menacing, there is a hint that he is planning something and he does not need to raise his voice. He said very quietly that the windmill was nonsense and that he advised nobody to vote for it, and promptly sat down again.
The way Napoleon makes his point during meetings carries threat.
