Sidious’
Power is the only thing keeping Anakin in check.
Lawrence Kasdan; Star Wars: The Annotated Screenplays (p. 265) wrote:
"My sense of the relationship is that the Emperor is much more powerful than Vader and that Vader is very much intimidated by him. Vader has dignity, but the Emperor in Jedi really has all the power."
The Darkside is what’s keeping Vader alive. When Vader saves Luke, he forfeits his life.
I think Palpatine will explain this during the “birth” scene. He will tell Anakin that if he turns from the Darkside, it will no longer sustain him. Anakin’s response may be: “I don’t fear the Darkside; I embrace it.”
