When an electric field is applied to the liquid crystals the twisted structure gets distorted to varying degrees depending on the voltage applied. The change in the structure subsequently causes a change in how drastically the light is rotated. This ultimately decides how much light will pass through the filters and determine the brightness of the sub-pixel.
When light passes through the first polarizing filter, half of it is blocked. The remaining light normally gets rotated by the liquid crystals and can pass through the second perpendicular polarizing filter. If it isn’t rotated at all, then no light will pass.
In small LCD devices, such as watches with 7-segment displays, it is possible to have a direct connection to control each segment.
This is not feasible for larger displays, like colour monitors. Millions of individual connections would be required. Therefore the pixels must be controlled by a multiplex array. Multiplexing reduces the connections required substantially.
There are associated problems with multiplexing since some of the inactive pixels will still be effected by the voltage running through their row or column. This was solved through “switching” where thin film transistors are used in the multiplex array, ensuring only activated pixels are effected.