"Flying down three thousands of feet" is a detailed description of the waterfall at close range. "Flying Flow" shows that waterfalls come out in the air and spew out. "Straight down" not only describes the steepness of the rock wall, but also describes the rapidity of the water flow. "Three thousands of feet" is an exaggeration, describing the towering mountains.
In this way, the poet felt that the majestic momentum of the waterfall had not been fully displayed, so he went on to write a sentence, "It is suspected that the Milky Way has fallen for nine days." It is said that this waterfall "flew down", which makes people suspect that the Milky Way poured down from nine days. A "doubt" is ethereal and lively. If it is true, it introduces reverie and adds the magical color of the waterfall.