How the multiple content databases will works for the Sharepoint webapplication

Hi,

I have web application with one content database. Due to some space issues I am planning to add one more content database for the web application. 

Here is my questions,

1) how the content will manage in between these two content Data Bases?

2) When the content will start moving to the new content database?

Any help would be greatly appreciated.

Regards,

Saya