Compression force operates down the length of the flake, stiffening it, and shearing force separates the flake from the core; these two forces operate at right angles to each other, and when approximately balanced, the crack travels parallel to the core face.  In this case, the ‘force angle’ driving the crack is aligned with the face of the core.  With feather terminations, the fracture front remains in balance until it cleanly exits the stone, thus creating the sharp edge that makes the flake so useful as a cutting tool.