Why Won't a Universal Motor Run if Triac Is Shorted Open in Speed Controller Circuit?

On page 5 - 150, there is a circuit diagram of a speed controller for a universal motor. The controller has a potentiometer feeding a capacitor that will turn on a diac at 30 V which will activate the triac. My question is, since the RC circuit is in series with the motor why will the motor not run if the triac is shorted open.

http://books.google.com/books?id=vmg1UKsTntAC...th+speed+control+not+running&output=html_text