Taking the simple approach - the expanding burning gases will take the path of least resistance, which in this case is out the rear of the engine, since the front is 'blocked' by the highly compressed air entering the engine. The difference in pressure at the nozzle exit (and fan-bypass exhaust) is the thrust produced.
The amount of fuel burnt is proportional to the additional energy imparted to the airflow (less internal friction loss, etc).
-GY