I'm not sure that I understand what type of instruction you are looking for. Triggers are just used by the server behaviors for determining when to execute the code for that server behavior. So if you have a special trigger for the insert then only when that triggers conditions are met will the insert occur. In this context what would you like the trigger to do for you?
If you are looking to add in functionality on change of a select list this would be a client side behavior, not a server behavior. You should be able to select the element and go to Tag > behaviors to set an action for the event of that form field. You will not be able to trigger server behaviors directly like this though.