The Shutter Streak tool emulates the look of light leaking into the bottom and top of the film because of a misaligned film gate.

Size: The horizontal scale of the streak as measured from the midpoint of the frame to the top and bottom edges.

Boost: Increases or decreases the overall exposure change of just the streaks and not the underlying image.

Falloff: Controls the softness of the streak as it approaches the center of the image. Positive values cause greater falloff and less visible streak, while negative values emphasize the streak in the frame.