Why
During a fight, opponents always made sure to have a film of qi surrounding their body, and the work of this film is to prevent the opponent's qi from getting into their body.
Treuchi was an experienced fighter and so he had made sure to enforce the location where Seth's palm had hit him with a strong layer of qi, but now he could feel traces of a powerful foreign qi rampaging within his body, and though surprised that Seth's qi had managed to make its way through his defense and penetrate into his body, Treuchi quickly reacted and diverted some of his own qi to expel out the foreign qi within him, but he was shocked to find that the foreign qi easily overcame his own qi and continued spreading calamity within him, and before he knew it, his body was wracked in pain.