When does OnSelectedIndexChanged happen?

If a form is submitted that has a list control (in my case, a RadioButtonList) with a OnSelectedIndexChanged event handler, when does is the "check" done for a change in index? I'm trying to figure out the order in which events fire.  After my form is submitted the form data become parameters to run a SQL SELECT query.  I'm trying to figure out if the event handler for the OnSelectedIndexChanged is supposed to happen BEFORE or AFTER the stored procedure would be run.  The microsoft website just says that it checks for index change on trips to the server.

 

Thanks
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