Visiting this page triggers the server to archive its current data for download. For performance reasons, the download won't begin while any users are actively playing games. The process takes ~3-5 minutes. Note that AWS bandwidth throttling can cause this process to fail. You can ssh into the server directly and then SCP the data (see config/server-config.json for the data_prefix which is where all data is stored).
The download will begin automatically once the data is ready.