I've noticed the makeshift crop top thing from other countries as well, not just China (I imagine this is from China). In the Philippines, when local thugs would challenge each other, they would raise their shirts to make that kind of makeshift crop top when trying to intimidate rivals or people. Some just completely throw their shirts down. They'd get into a brawl with their crop tops and then sometimes stab each other's exposed bodies. I guess that crop top move is just posturing as if to say "Hey, I'm a tough guy, look at this body!".