"I'll give you nary a thought," Porthos protests, unable to completely stifle his grin as he does so, and it is a lot more difficult to get out from under someone when you weigh so much less than them, he is now finding out, much to his chagrin. His wrists can't escape Aramis forever, and the buck of his hips are useless at throwing him off so far.

So he stops fighting and just lies there, hoping to lure Aramis into a sense of safety and throw him off then. Of course, in a real fight, he'd have simply opted for a headbutt, but even he wouldn't go there for a friendly brawl. "See? Everything's impossible in this body," he states, to add to the pretense that he is giving up.