How did the pigs use Boxer's death to get the animals to work harder?

The pigs used Boxer's death as a means for getting the animals to work harder by using his life as an example for the degree of work the pigs expected from the animals. They used Boxer's slogans for his own work and wanted the animals to apply it to their own work. They tried to make it seem to the animals that they were honoring Boxer's memory by subscribing to his slogans. Napoleon himself addressed the animals at the close of chapter nine and he reminded the animals during his speech that Boxer's favorite slogans were, "'I will work harder' and 'Comrade Napoleon is always right'". He made it a point to tell the animals that it would be in their best interest to adopt these slogans as their own.