Porthos is very aware that he should be on the ground already, and not because of his lack of balance. Blocking Aramis's attack is incredibly taxing, with the weight already behind it, and his face contorts as he gives it his all, then thinks better than to try and push back against it, and sidesteps it instead, of a sudden, hoping to catch his friend off guard. He's seen lighter fighters than him pull off that move times and times again, and Aramis's lack of familiarity with the heavier blade might just make it work.