The work load described in this quote shows that the farm is going back to the kind of involuntary toil that was characteristic of the animals' lives under the old regime of Farmer Jones. This is not the free and enthusiastic pulling together of the old days, when the animals believed they would all share in the farm's prosperity. This work load is imposed by the pigs: as the quote says "Napoleon announced . . ."
It is worth noting that what the animals undergo is similar to the heavy work shifts imposed under Stalin in the Soviet Union in the 1930s. These shifts were called "shock work" and were meant to boost productivity levels so that the government could achieve the unrealistic goals of its Five-Year Plans. As with the animals, the workers were given no extra pay or food.
We see too the manipulation of words, a skill that the pigs are adept at. The pigs pretend this is all "voluntary," but by threatening to cut the animals already very meager rations in half, the pigs are essentially saying that any animal that doesn't do the extra work will face near starvation.