What's not to understand? The dialogue explained that MC used his Transcendent Perception skill to dodge the incoming attacks from Tanaka. All his senses were sealed, but his body moved automatically (involuntary movement) in reaction to the danger from Tanaka's attacks.
Since Tanaka is so fast and strong, Nagi's defensive movements were quite strenuous and had a severe blowback, like a whiplash effect, on Nagi's body, so that's why he felt like shit afterward (sore all over and nauseous).
Could they have shown Nagi dodging while Tanaka attacked him? Yes. And certainly, that would have made more sense, but maybe they just wanted to show it as if from Nagi's perspective, which is why he narrated what his body was doing on his own, like his head moving side to side, and his arms moving up, and etc. And then showed and explained the aftermath with dialogue.
With that said, I do think the writer is trying to cram too much into too few panels, but that may be out of their hands. It could be someone like the editor telling them to have X amount of exposition in only Y amount of pages, or something like that.