How Can I Make A Web Form "Expand" to Include Other Options??

I've always designed my web forms with ASP.Net and I feel like I've been able to turn out some pretty viable solutions. One thing that really annoyed me were post backs that resulted when an option was selected from a drop down list.

Obviously I need the fact that an item was selected and/or changed to be recognized for the sake of form validation but the entire screen flashing for a second during the process just looks unprofessional.

I recently completed the Experts Exchange End of the Year Survey. Within that survey if you selected "Other" the form expanded and another text box appeared asking you to explain further.

Can some explain how this was done and provide an example?

Also, is this possible with .Net?

Again, I just want the option of entries being recognized without the annoying flashing going on.

Thanks in advance,

JB