China's new border rules take effect September 15, and experts in Taipei warn they give Beijing a tool to detain people, seize phone data, and even spy on Taiwan in reverse — bad news for Taiwanese business people and tech workers.

• The rules let border officers demand documents, files, and electronic data, turning phones and laptops into search targets.
• Taiwanese are treated as Chinese citizens, so past criticism of Beijing — made anywhere in the world — could trigger an exit ban.
• A "let them in, then block them from leaving" trap is the biggest worry, stacking on top of China's 2023 anti-espionage law.
• Beijing is believed to be building a database covering every Taiwanese person, linking ID numbers to phones and social networks, run by AI.
• Tech workers and Taiwanese business owners who partner with US or European firms are seen as prime targets for questioning about their work.

Outlook: Taiwan's travel warning for China stays at orange for now, but pressure to raise it will grow if detentions start.