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https://www.livescience.com/20104-boogie-board-ewriter-nsf-bts.html

How it works

The eWriter works on the principle of anisotropic flow, a unique feature of cholesteric liquid crystals, in which crystals flow at different rates, depending on the direction of pressure being applied. When a thin film of cholesteric liquid crystal is sandwiched between two sheets of specialized plastic, anisotropic flow causes the molecules to order themselves to reflect light in those places where a stylus touches the plastic sheet.

The liquid crystals are surrounded by polymer pillars, which control flow, resulting in excellent line sharpness. The written image is retained until electrically erased with the push of a button. In electronic erase, the flash of an electric field rearranges the molecules so that they are less reflective.