[Update 7/2: Changed "Commission" to "Committee" to reflect official translation] [# in brackets reference article numbers of the law]
The new Hong Kong national security law’s central aim is to make it easier for China’s central government to act more directly in Hong Kong to discover, prevent, and control perceived threats to national security. This is accomplished both through the central government establishing its own institutions in Hong Kong, and also by its taking greater control of Hong Kong authorities.
The centerpiece of the law’s plan for this tighter engagement is the creation of the Central Government Office for the Preservation of National Security in Hong Kong (the Office) and a local counterpart, the Hong Kong Committee on Preserving National Security (the Committee), jointly responsible for national security efforts in Hong Kong.
The Committee is led by Hong Kong’s Chief Executive and consists of relevant ranking leaders, but is expressly accountable to the central government.  It is responsible for drafting local national security policies and laws, planning, and coordinating national security actions, but its work is largely to remain undisclosed.  A national security adviser appointed by the central government is to observe the Committee’s meetings. 
While that law does not specify the membership of the Office, the National People’s Congress’s formal decision to draft the law said that government organs “related to the preservation of national security” should be authorized to create offices in Hong Kong. This undoubtedly includes personnel under both the Ministry of State Security (national security) and the Ministry of Public Security, an idea that is confirmed by the Office’s having criminal investigation and intelligence gathering duties that these ministries are uniquely equipped for
Two-Tiered Criminal Procedure for National Security: Bad and Worse
The law creates a special bifurcated criminal procedure for national security cases that illustrates the law’s approach of further controlled local forces and direct central government action. Here, most cases are handled by local authorities under local law with central government oversight, but a parallel path is created for mainland authorities to take control and address the case applying mainland law.
Even on the local law path, cases are to be handled by a newly formed national security police team with greatly expanded investigatory powers of search and surveillance , the leader of which is appointed by Hong Kong’s Chief Executive in consultation with the central government Office . Similarly, the Chief Executive is to designate the judges and prosecutors eligible to handle national security cases 
The right to a jury trial remains- unless the Secretary of Justice waives it due to involvement of foreign elements, juror safety, or to protect state secrets; the possibility of bail remains- if the judge holds that there are grounds to believe they won’t commit further offenses , and trials will be open, except to protect state secrets or public order. 
The Central government can authorize its National Security Office to take direct jurisdiction at the request of either Hong Kong’s Committee or the Office itself in any of three broad situations: 1) when the involvement of foreign forces makes it hard for Hong Kong to handle the case; 2) The Hong Kong government can’t effectively implement the law, and 3) when faced with significant real threats to national security. 
In these cases, the Office will conduct the investigation itself, and prosecutors and judges will be selected by the mainland’s Supreme Court and Procuratorate.  From investigation through the enforcement of penalties, the mainland Criminal Procedure Law will govern, including limiting access to an attorney to after the first interrogation or the imposition of ‘compulsory measures’. [57,58].
The work of the Office is told to comply with Hong Kong’s laws as well as national laws, under supervision of the mainland procuratorate, but is not subject to Hong Kong law or interference by local authorities. 
Both the local and central government jurisdictional paths apply only to certain types of crimes, namely: Separatism (secession), Subversion of State Power, Terrorism, and Collusion with Foreign or Taiwan Forces. The law breaks down the elements for each of these offenses, some of which include only speech or advocacy, and others rely on subjective assessments that may be difficult to apply. Terrorism, for example, includes conduct that is violent or destructive, but only when it is meant to further a political agenda through coercion or intimidation, a more subjective standard.
National security prosecutions must be approved by the Secretary of Justice , and the Chief Executive has the right to issue binding declarations to courts regarding a case’s designation as involving national security , but what happens in the earlier phases of criminal investigation to sort jurisdiction among three different law enforcement groups (HK, HK National Security, the Office) each acting under different criminal procedures.
It isn’t yet clear how frequently this law will be applied beyond addressing the ongoing protests, but the chilling effect may have as stark an impact as actual enforcement. The authorities and the text itself offer assurances that the law will be used sparingly and civil rights will be respected, but it also calls for closer scrutiny of schools, social organizations, and the internet. The law’s message is clear that the central government authorities are in Hong Kong with sweeping authority, and are watching.