Meaning Hong Kong would technically no longer be part of China, and would revert to British sovereignty.
Meaning, if they pass this law and take over defense/security in HK, they are essentially violating the Hong Kong Basic Law (Constitution) and invading what would then be (through their actions) a foreign country.
Defer to international lawyers on the technicalities, but that's how I've seen this read.
https://en.wikipedia.org/wiki/Sino-British_Joint_Declaration