crathens makes a request to the /catalog endpoint of any Athens server (that you define in the GOPROXY env var) and, for each module in the returned list that has github.com at the beginning of the path:
Requests the list of tags for the corresponding GH repository
Sends a /{module}/@v/{tag}.info request to Athens for each tag in the response body
The crathens daemon rate limits both steps to prevent flooding the GH API (and getting rate limited) and to prevent overloading the Athens server at GOPROXY.