In the old days we used fscommand and getUrl() to do some "dirty-work" from Flash to the webserver. Nowadays Flash has an external interface you can use but basically does the same thing.
What you are looking to do is have Flash call JavaScript, and then the JavaScript calls your ASPX and/or an AJAX-like service. This is the exact same way that the classic versions of .Net (1.1, 2.0) used the __doPostback() event to post back to the server.
http://www.adobe.com/devnet/flash/articles/external_interface.html[
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