the fire-ring is the specific metal ring incorporated into the headgsasket, whose purpose is to keep the pressures created in the combustion chamber from escaping.
Sometimes the pressure can get so high that the fire ring gets slowly nudged towards the most weakest direction. Eventually it breakes and blows out often towards the valley . This leaves a direct path from the chamber to the valley. When running, the valley is chock full of oil in motion, and some of it is airborn. It is then easier for the piston on the intake stroke, to pull in air from there especially with the throttle closed or nearly so. Of course with the torn fire ring, that cylinder will then blow it all back on the compression stroke, the engine idles rough, and loses a bunch of power.
