This new property (*not* event), allows you to wire up clientside code to be executed before the server-side onclick is handled. In the old days we had to do this manually. For example, this is how you would hook up a simple confirmation dialog:
<script language="jscript" type="text/jscript">
if (!window.confirm("Are you sure?"))
window.event.returnValue = false;
This allows you to cancel the server-side event if the user says nope! Cool!