All subtasks start with the "subtask". Task actions are usually data events. I can't remember if OnTaskAssignmentChanged was a built-in ruleset or not but if it's not there do the following:
1. Under the ruleset listings for IM, click Add Ruleset.
2. Name the ruleset whatever you want I named it OnTaskAssignmentChanged.
3. Change the ruleset type to Data event.
4. Change event dropdown to TaskAssignmentChanged.
5. Click Save.
Add a rule to the new rulset and give it the following:
Condition: Task name starts with "subtask" (without quotes)
Action: Send email to new task assignee using "your email template"
That should get you what you're looking for.