Now slowly move it away from the heads and unplug it when it is about four feet away. DO NOT use a demagger with a switch because if the switch is released or fails near the heads, you could have a set of magnetized heads that will be difficult to demag without a much more powerful demagnetizer. They are magnetized by the collapsing field, which could be quite intense depending where in the AC cycle the power stopped. For the really paranoid, you could use a computer UPS to power the demagger, to insure clean, uninterrupted power just in case the power failed while demagging. One of the myths is how easy it is to demagnetize tape.
Tape is very resistant to demagnetizing.
Once for my own education, I took a tape and played it, while moving a demagnetizer up to the tape. While this was very uncontrolled and unscientific test, I could detect no appreciable difference until the demagnetizer was within a couple of inches of the tape. There is no need to remove tapes from the control room when demagging, as I have seen done. If tape was that easy to erase, the fields from all the transformers in all the gear in the control room would wipe the tape as you walked in. If bulk erasing tape, don't just set the tape on the bulk eraser and turn it on, erase the tape and turn it off.