The technique you refer to is actually relatively straightforward, and a great way to solve very strong color casts in an image (which is common with old faded photos, for example). The technique can be employed as follows:

1) Create a duplicate of the Background image layer by dragging it to the Create a New Layer button (the blank sheet of paper) at the bottom of the Layers palette.
2) Select Filter > Blur > Average to convert this layer to a single color representing the color cast.
3) Choose Image > Adjustments > Invert to change the color of this layer to its opposite (the color needed to compensate for the color cast).
4) Set the blend mode for this layer to Color (at the top-left of the Layers palette) so it will only affect the color of the underlying image layer).
5) Reduce the Opacity at the top-right of the Layers palette to the value that best neutralizes the color cast.

That's it! The result is an almost magical way to resolve very strong color casts in an image.