The US is considering some new rules that could cause a stock market calamity in China. The government is considering putting new restrictions on US capital flowing to the Chinese mainland. The move is considered the third and worst-case-scenario stage for Chinese markets in the current trade war. In particular, the big risk is that MSCI de-lists Chinese stocks from its broader indexes, meaning all that capital would need to be pulled out. That amount is currently around $50 to $60 bn.
FINSUM: This is not hugely massive, but it is certainly enough to hurt markets on a technical front, but perhaps even more from a perception angle.