The function of eye consciousness is only to see visible forms, not to ascertain physical gestures or movements. However, succeeding mental processes occur so quickly that ordinary people think that they see as if with their real eyes, the movement known by the succeeding mental process of investigation.
For example, when we see a hand moving, our eye-consciousness sees only the visible form. It is not able to know that it is a hand or that it is moving. the mind moves very fast, however, so the movement that the succeeding mind of investigation knows is taken to have been seen with the eyes. Ordinary people can not distinguish between preceding and succeeding mental processes. On the other hand, a meditator who has practiced insight proficiently can recognize the mental process of seeing visible form as distinct from the subsequent mental processesthat know it as a hand and movement.
The Pali text give the example of a swinging torch to clarify this point. If a lit torch is twirled in the darkness, it will appear as a solid ring of fire to anyone watching. If it is swung in a linear or triangualr pattern, then it will appear as a line or a triangle. In reality, there is no circular, linear or triangular shape to the fire, only the red visible form that can be seen moving from place to place as the fire passes. In reality, it is the succeeding mental processes that merge the visible forms that appear in different places and interpret them as a circle of fire and so on. This is the actual example from the Pali texts.
Manual of Insight, Page 95