How does NCR paper work?

Taking a two-part set as an example, the top sheet has a Coating on the Back (the paper is specified as ‘CB’) and the bottom sheet has a Coating on the Front (‘CF’), and when placed together and impacted (using a pen or dot matrix printer), the two surfaces react and produce an image. If the sheets are upside down or collated the wrong way round the process doesn’t work. If you have a three-part (or greater) set the middle sheet(s) are Coated on the Front and Back (‘CFB’).