To answer this question I need to know wich version of arcgis you are using (9.3 or 10.X)?
Are you trying to do this in an Arcgis engine application or in a custom tool for arcmap.
There are a few diferences, but I think I have a few examples.
I've been struggling with this in the past and it took me a while to figure this out.
If you can give me some more details on what you are trying to do to make a feature or geometry flash,
I can help you with that.
Regards
Gwen.