Hi @robertz,
I apologize for the delay in response and would like to try and provide an answer for future users who have a similar question.
Pre and post registration hooks run asynchronously, but are triggered before a rule. To visualize I can link you to this document:
Rules are going to run during every authentication and hooks are only going to run during their specific event, before or after registration or during a client credentials grant, depending on the type of hook you have set up.
Thanks,
Dan