If （衝撃、そして背後に流れていく視界）の中～ means "amid 衝撃 and X視界 (field of sight), where X is 視界's predicate, then what does 背後に流れていく視界 mean? Looking back? Literally seeing what's behind? The "field of sight streaming (towards) back/behind" doesn't make sense to me when translated literally.
As far as I know, 背後に流れる視界 has nothing to do with any existing Japanese idioms or slang expressions, and this phrase is kind of puzzling to the eyes of native speakers, too, if we have to interpret this logically. But we can stick to the literal translation ("the field of view flowing rearwards") and speculate what it means.
I may be wrong, but my impression after seeing this sentence is that he is probably experiencing some kind of "shock wave" caused by his enemy in front of him, like in this video. In other words, this sentence is probably trying to say "(彼の)視界の中のすべてのものが背後に流れていく中" (lit. while everything is flowing rearwards in (his) field of view). I don't think he was trying to look back deliberately.
I would intepret it as "(all the things in) his vision are flowing past him, towards his rear".
So maybe he was moving forward, or from the impact, everything around his was blown past him in a shockwave.
This scene came to mind. https://youtu.be/DpezTC6-aZ4?t=259
