$MMX is an internal variable that returns the value of the last amount of x-axis mouse movement.
If the player moves their mouse to the right, this value would hold a positive value for that frame. If the player moves their mouse to the left, this value would hold a negative value for that frame.
It is used like any other variable, except you cannot use "setvar" on it, since it is an internal variable and the setting is handled by the engine.
If you would like to see an example of its usage, you can open up the "motionblur.fpi" script which should have been included with one of your PB downloads. That script utilizes the mouse movement to alter the motionblur shader for effects.
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