In the end of Avengers (2012) movie,
BLACK WIDOW: Doctor.
SELVIG: Loki's scepter, the energy...the Tesseract can't fight. You can't protect against yourself.
BLACK WIDOW: It's not your fault. You didn't know what you were doing.
SELVIG: Well, actually I think I did. I built in a safety to cut the power source.
TONY: Well, that sounds exhausting. The rest of the raw materials, Agent Barton can get his hands on pretty easily. Only major component he still needs is a power source. A high energy density, something to kick start the cube.
Well, Selvig used Stark Tower's arc reactor as power source, but it was only necessary for booting the portal device as Tony predicted.
JARVIS: Sir, I took off the arc reactor. The device is already self-sustained.
Which power source did Dr. Selvig suggest to cut? Was he talking about cutting the cube's connection with rest of the portal device?