Flakes are initiated conchoidally through the formation of a Hertzian cone.  In Hertzian cone formation, the area in direct contact with the indentor—usually less than 2-3 mm in diameter—is depressed until the stone fails and a crack starts.  The tiny circular crack is sometimes visible in fine-grained stone, and is referred to as a ‘ring crack’.  Often only half of the circle is visible where it is exposed when the flake detaches from the core.  This semi-circular feature is referred to as the ‘umbo’. 

The presence of the ring crack or umbo defines the PFA.  This attribute is particularly important because certain metrical attributes are measured in reference to it.  For instance, the distance from the PFA to the platform edge records how far back from the core edge the stoneworker delivered the blow, which in turn provides information on technique and precision.  Bend-initiated flakes do not have a Hertzian cone, and they therefore do not have ring cracks or umboes.  By convention, the PFA on bend-initiated flakes is usually taken as the centre of the platform edge.