Look if he holds back then Isidor's going to make him look like a chump...is what Jim would have said if a spark of pain wasn't jolting through his arm. If his body wasn't twisting with Isidor's steady weight guiding his momentum when his foot runs into hers and he's halted from going forward.

Pain and getting the air knocked out of him when he hits the floor rouse something in him that wasn't there before. He lashes out in a kick before Isidor can try to pin him down. He doesn't expect to get her leg out from underneath her with this, but he does want to buy himself time and distance to get back to his feet.

This is still a simulation. It's still not Real. But Jim doesn't want to go down without a fight here either and it's starting to show. He won't have time to do much more than get up and throw up an arm to try and block when Isidor inevitably closes the distance again while he's still sucking in air.