This article lists some key provisions from the Chinese Communist Party's security law: https://www.bbc.com/news/world-asia-china-52765838
Some real kickers in here:
* Beijing will establish a new security office in Hong Kong, with its own law enforcement personnel - neither of which would come under the local authority's jurisdiction
* This office can send some cases to be tried in mainland China - but Beijing has said it will only have that power over a "tiny number" of cases
* Beijing will have power over how the law should be interpreted, not any Hong Kong judicial or policy body. If the law conflicts with any Hong Kong law, the Beijing law takes priority