Before the hapless general could die, Vader s superior orders the man released. And abruptly, with a hand gesture, Vader releases the man, who then slumps on the conference table gasping. Strength comes from the force within, or so the scene from the movie teaches. Take your pick. The movie has its own explanation.
Eastern mystics have a similar one.
As Luke is learning to use the Force, the audience is treated to explanations of its nature. It permeates all things. Living things generate it. It flows through you and controls you, but you can control it with your mind and your feelings. It is a power within.