Synonyms for the phrase "zip down" include "fly down", "whiz down", and "race down". Each of these phrases depicts swift and rapid movement, typically associated with descending or traveling downward at a high speed. "Fly down" implies a smooth and graceful descent, evoking the image of a bird gliding effortlessly through the air. "Whiz down" suggests a quick and energetic descent, often used to describe movement akin to a speeding object. On the other hand, "race down" conveys a sense of competition or urgency, portraying a fast and competitive descent. These synonyms offer a variety of descriptions for the concept of moving rapidly downward.