Bleach bypass, also known as skip bleach or silver retention, is an optical effect which entails either the partial or complete skipping of the bleaching function during the processing of a color film. By doing this, the silver is retained in the emulsion along with the color dyes. The result is a black and white image over a color image. The images usually have reduced saturation and exposure latitude, along with increased contrast and graininess. It usually is used to maximal effect in conjunction with a one-stop underexposure. (Wikipedia)
In the digital era a bleach bypass effect can be achieved with the touch of a button.