"A quest to identify protesters and police officers has people in both groups desperate to protect their anonymity... Mr. Cheung, a skinny 29-year-old, was grabbed at a mall around noon on July 18, according to his account. Four plainclothes officers waited for him to unlock his phone and then jumped on him, trying to pry it out of his hands... They grabbed his jaw to force his head in front of his iPhone. They slapped his face. They shouted, “Wake up!” They pried open his eyes. It all failed: Mr. Cheung had disabled his phone’s facial-recognition login with a quick button mash as soon as they grabbed him."