Basically (very basically) the toroidal vortex is the air that is not used of for combustion - it is designed to swirl around the outer edge of the combustion chamber to keep the flame path away from the combustion liner.
If the toroidal vortex flow is disturbed (blocked or damaged guide vanes etc) then the flame path can touch the combustion liner and eventually burn through.
I seem to remember the Rolls Royce Engine book has some decent diagrams of the airflow in the combustion zone.