The Directional Blur effect gives a layer the illusion of motion.
Andrew Kramer provides a video tutorial on his Video Copilot website in which he demonstrates the use of the Directional Blur effect to make a gun slide look as if it is moving quickly.
Direction: The direction of the blur. The blur is applied equally on either side of the center of a pixel; therefore, a setting of 180° and a setting of 0° produce the same results.
Taken from the adobe help site. https://helpx.adobe.com/after-effects/using/blur-sharpen-effects.html#directional_blur_effect