Hi,

approach using display BR + onLoad client script on incident_task

1) Create a Display Business Rule on your Incident Task Table and use the script mentioned below:

(function executeRule(current, previous /*null when async*/) {

	// Add your code here
	g_scratchpad.shortDescription = current.parent.short_description;
	g_scratchpad.description = current.parent.description;

})(current, previous);

 2) Write an On Load Client Script on Incident Task Table with below Code:

function onLoad() {
   //Type appropriate comment here, and begin script below

   g_form.setValue('short_description', g_scratchpad.shortDescription);
   g_form.setValue('description', g_scratchpad.description);

}

Regards
Ankur