Russia would most likely resort to nuclear weapons in that case.
They are aware of China's Little Green Men in the East - but China is opting for squatting at this point, not acting like the Russian method, and Xi's cash has kept the Putin kleptocracy afloat for the last 3 years - so Russia ignores it. But armed incursion with insurrection would be a red line that Putin most likely would use Nuclear Weapons for (as he can't use his army).
I think Taiwan originally was planned as a way to 'blood' the Chinese Army before ay conflict with Russia - but Ukraine has shown how costly an opposed invasion can be, and Xi doesn't want his legacy to be known as the Folly of the Chinese Putin...