Vox recently made a good video on the topic [1].
In 1984, Beijing agreed with Britain that after the handoff in 1997, China would respect Hong Kong’s political system [2]. Hence, Deng Xiaoping’s “one country, two systems” [3].
Beijing is breaking that agreement. It already tried abduction [4]. Now it wants to be able to extradite anyone to China to be tried in Chinese courts.
This is problematic. Hong Kong, like Japan, Taiwan, Britain and America, has an independent judiciary where the government must prove its case. So if a dissident is tried in Hong Kong, prosecutors have to prove their case in a relatively fair court.
Chinese courts are party instruments. (Consider that there is no way to enforce Hong Kong’s rights under the 1984 agreement.) Under the proposed legislation, said dissident would be shipped to China where a rubber-stamp conviction could be sought.
[1] https://m.youtube.com/watch?v=MQyxG4vTyZ8
[2] https://en.m.wikipedia.org/wiki/Transfer_of_sovereignty_over...
[3] https://en.m.wikipedia.org/wiki/One_country,_two_systems
[4] https://en.m.wikipedia.org/wiki/Causeway_Bay_Books_disappear...