Since the WuXi Finger Hold was made up by masters to scare their students, it must cause a lot of stress to the person it is used on (if that person didn't know that it actually doesn't really do anything). After Po does this move on Tai Lung there is a golden flash followed by what looks like yellow haze right? My theory is that Tai Lung let out a "big one" if you know what I mean. He looked as though he was pretty scared (thus under stress), and when someone is under a lot of stress gas tends to build up in one's intestinal track. The yellow haze also sort of backs this claim as generally in cartoons the color of flatulence would be green or yellow. As for the smell... I'll leave you readers to decide on that.