Chen Quanguo, the official directing China’s clampdown in its restive Xinjiang region, has emerged as a pioneer of aggressive policing techniques—setting the tone for the country’s shift toward harsher, technology-driven authoritarian rule.

As Communist Party chief for Xinjiang for the past 2½ years, Mr. Chen has created a policing regime unmatched in scale and sophistication. He brought some of his techniques from earlier provincial posts, including in Tibet, and expanded them with new technology and tactics.

He...