When does Dash ping server?

I am trying to optimize the performance of a Dash site and wondering if there are resources that describe which processes are happening in the browser, which are happening on the server, and when data is being transferred between the two?

For example, when I have a callback is it reloading the the whole page or simply sending a signal to the backed to execute a python script and send back the results without reloading page?

Thanks!
Isaac