I vaguely know that HTTP Post does not send all the data (hidden fields?)
along with the request.
I would like to know the stage in ASP.Net application life cycle where the
POST data is read - is it in HttpApplication.Init or at the time when
viewstate / postback data is loaded? I am not able to find this type of info
on MSDN or other sites.

It would be great if someone also explains the difference between GET and
POST with reference to the ASP.Net application life cycle.