The polarization state of transmitted light is linked to liquid crystal (LC) molecular distribution. The dynamic behavior of a twisted nematic LC molecule is measured with a home-built 10 kHz snapshot polarimeter. Only the transient molecule rotations are observed when the external voltage changes, and the molecules return to their original orientations quickly even when high voltage is applied. Our observations cannot be attributed to the traditional electro-optic effect. The invalidation of the static external field indicates the shielding effect of redistributing impurity ions in an LC cell.