Webforms is a simple single request and single response. The server can not update the browser after the page response. To do what you want, the timer must be in the browser, and make a new request. This is done with the refresh meta tag or JavaScript.
The webform event will not happen until the browser posts the form. This will cause the page processing and events to be called. I believe the dropdown has an auto post back feature, which generates JavaScript to do a form submit on change. This will send the from data to the server, the server will process the form data, and create a new page which the browser displays.
note: this is a webform question and should be asked in the asp.net web forms section, not the newer asp.net core