wikipedia

Early autopens used a plastic matrix of the original signature which is a channel cut into an engraved plate in the shape of a wheel. A stylus driven by an electric motor followed the x- and y-axis of a profile or shape engraved in the plate (which is why it is called a matrix). The stylus is mechanically connected to an arm which can hold almost any common writing instrument, so the favourite pen and ink can be used to suggest authenticity. The autopen signature is made with even pressure (and indentation in the paper), which is how these machines are distinguishable from original handwriting where the pressure varies.