You know, like double checks or three-sheet invoices where you keep the white copy, the pink copy gets attached to the package and the yellow copy goes inter-office mail to Jerry in billing? How do the words appear on the bottom copy/ies?
There are microencapsulated dyes deposited on the back surface of the paper. On the front surfaces of the copy forms is deposited a reactive clay powder. Pressure from the pen breaks open the microcapsules, and the released dye reacts with the clay to form a permanent, dry mark.
Wikipedia has a nice diagram to illustrate, in case my description wasn’t clear enough.