Redscale is a technique where film is exposed on the wrong side — rather than having the light hit the emulsion directly, you expose the film through the non-sensitive side.
The name “redscale” comes because there is a strong color shift to red due to the red-sensitive layer of the film being exposed first, rather than last (the red layer is normally the bottom layer in C-41 (color print) film). All layers are sensitive to blue light, so normally the blue layer is on top, followed by a filter. In this technique, blue light exposes the layers containing red and green dyes, but the layer containing blue dye is left unexposed due to the filter. [#]
The two main ways for doing this are loading the film upside down (if your camera allows it), or by purchasing film that has been “converted” already. A third way is to make DIY redscale film by going into a darkroom, pulling out the film, cutting it, flipping it, taping it back together, and then winding it back into the canister. Messy, but it works!