Chen Mu had entered the woods in a flash, having just discovered someone observing them from the dark. When the tailless shuttle left his hand, it had hit the man in the shoulder. The tailless shuttle had a confusing peculiarity, which was that it's flight would get faster the father it went. At the outset, the enemy would have gotten its initial speed, which wasn't very fast, but its tail would be continually dissolving as its speed got faster and faster, until it had reached a frightening speed.
That guy in hiding had taking a hit because of that. He had originally thought he could dodge it, but hadn't realized that the shuttle's speed would increase, so he was surprised, and he couldn't dodge it and got hit in the shoulder.