trying to criticize writing on the fly is possibly the singl

trying to criticize writing on the fly is possibly the single greatsest barrier to writing that most of us encounter.
这句话里,trying to criticize writing on the 是说匆忙的批判性写作还是说,在写作的过程中尝试去批判,就是trying to criticize when writing on the fly.省略when呢?
就是这句前面看不懂,顺便帮我分析一下里面涉及的语法,
匆忙的批判性写作,还是说,在写作的过程中尝试去批判.
在这里强调一下强调，我想知道为什么