The FXScript Reference

This function is kind of slow and clumsy. Most digital imaging motion blurs work by shifting the source image equal distances from the center. For whatever reason, Final Cut Pro's Motion Blur only moves in one direction. To simulate the effect of a more traditional directional/motion blur, this filter needs to be applied twice in opposite directions.

Steps sets how smooth the blur will be, increasing this value will add substantial render time to any filter using the MotionBlur function.

For pure vertical and horizontal blurs, a much better option is to use Blur or BlurChannel with a severe aspect ratio distortion.