I think that the most evident answer to this is that the animals wrestle power away from the humans. Mr. Jones' own inept leadership enables the animals to rise and overwhelm him to gain control of the farm. Power changes in this respect, from human to animal. However, once the pigs, in particular Napoleon, gain control, Orwell's genius is to show that their demonstration and exercise of power is no different than the humans. Using the tenets and philosophies of Old Major to gain power, the animals end up violating these ideals when the gain it. The control of the farm and the maintenance of their political power resides in acting as humans in animal form. It is in this vein that the pigs are able to broker deals and negotiations with humans, force loyalty from the animals through violence and force, engage in alcohol consumption, and present a vision of control that as just as strong hierarchy as when the humans were in control. In the end, when the animals look through the window and see no difference between the pigs and the humans, they are right in their vision. Power has changed in terms of human replaced by animal, but in the end, there is little difference in terms of the control and the structure of rule in which the animals, as a whole, lack any substantive notion of power.