Why does applying an electric field make a twisted-nematic LCD cell appear dark?

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liquid crystal Figure 1: Arrangements of molecules.

In a twisted-nematic cell, the device appears dark when an electric field is applied because the field disrupts the liquid crystal's ability to manipulate light.12

Normally, the liquid crystal molecules are arranged in a 90-degree twisted configuration that rotates the polarization of incoming light, allowing it to pass through the crossed polarizers and making the cell appear transparent. When an electric field is applied, it forces the liquid crystal molecules to reorient perpendicular to the substrate plates.12

In this untwisted state, the liquid crystal can no longer rotate the polarization of the light to match the orientation of the top polarizer. Consequently, the light is blocked by the crossed polarizers, causing the cell to appear dark.21