If you have some thing(Properties or functions) to call from the normal page to master page you normally call using type conversion or if you want to call some controls on the master page you achieve with FindControl method.
If you specify mastertype of the page level you can strongly call the [...]

ASP.NET 2.0 introduced the concept of cross-page postbacks where one page could postback information to a page other than itself. This is done by setting the PostBackUrl property of a button to the name of the page that the button should postback data to. Normally, the posted data can be accessed [...]