After Effect uses the luminance value of each pixel in the displacement map to calculate the displacement of the corresponding pixel in the effect layer, based on the maximum displacement amount.
In grayscale images, the luminance value range extends from 0 to 255, and it’s converted to a scale ranging from -1 to 1. A luminance value of 0 produces maximum backward displacement, meaning that pixels at the current time are replaced by pixels from a previous time. A luminance value of 255 produces maximum positive displacement, meaning that pixels at the current time are replaced by pixels at a future time. A luminance value of 128 produces no displacement. For other values in grayscale displacement maps, you can calculate the displacement amount, in seconds, using the following equation:
Displacement amount in seconds = maximum_displacement_time * (2 * (luminance_value – 128) / 256)