It was all so exciting, a song come to life; the clatter of swords, the
flicker of torchlight, banners dancing in the wind, horses snorting and
whinnying, the golden glow of sunrise slanting through the bars of the
portcullis as it jerked upward.
as it jerked upward中it指代的是什么呢，我认为it应该指代主语glow 但看这个句子意思好像指代的是portcullis，所以As 引导的时间状语从句主语可以和主句不一样么
