When using a DLP projector to play a movie there are black bars on the top and bottom (widescreen), when positioned correctly, these prisms remove the black bars by stretching/correcting the image with no distortion.
This come from your website is totally inaccurate. The projector
removes the black bars through a process we shall simply call "scaling". As a result, the geometry is affected (the image appears tall and thin as if vertically stretched). When the prisms are placed in the light path (with correct alignment) they optically restores the geometry through horizontal expansion, allowing the projection of films in cinema scope the way they were meant to be seen...