Taiwan is warning people to think twice before traveling to China, as new border rules take effect that could trap visitors inside the country — bad news for business travelers, researchers, and anyone with a public opinion on China.

• New rules starting September 15 let Chinese authorities demand personal digital data, including phone and account information, from people crossing the border.
• Officials can block someone from leaving over vague "national security" or "technology security" grounds, and do not have to explain why.
• People accused of breaking Chinese law while abroad can be barred from leaving for six months to three years once they enter.
• Taiwan's national security chief Wu Jaushieh calls this a shift from border control to political control, and a new legal tool for cross-border crackdowns.
• Scientists, business people, and researchers are seen as most at risk, since their knowledge makes them valuable targets.

Outlook: Expect more travel warnings and fewer Taiwanese and foreign business trips to China once the rules kick in next month.