I am giving you one example.
In Master Page:
Menu 1 -> A.aspx (Content Page)
Menu 2 -> B.aspx (Content Page)
Menu 3 -> C.aspx (Content Page)
Normally if we are using these concept we are just redirecting to that page with Master page content. My requirement is suppose I am clicking A.aspx window will loaded with some form A form details. Next I am clicking B.aspx that time my A.aspx should restore like our desktop application. So at a time I will work both A.aspx and B.aspx like Multitasking. I have refered this link http://output.jsbin.com/ehagoy/154 . I am planning to implement this concept in my project. If you have any idea please suggest how to do achieve.