Here can you find out more about it.
http://cdnsun.com/how-it-works
Or this is maybe covering what you asking about:
How Is Content Uploaded To CDN?
Your content is pulled from your website by our CDN servers automatically when it is requested by your visitors.
When some your visitor at some location requests your CDN content, the content is pulled from your server (if it is not already cached), served to the visitor and then cached at our CDN server. All additional requests in that location are served from that CDN server until the content expires.
Moreover you can prefetch your content to all CDN servers immediately using the client section or the API.