Bill Tindall
With the grain running uphill the forces on the blade tip pull it, and the plane sole, into the wood with considerable force, so long as the wear on the blade bottom hasn't destroyed clearance. One could speculate how a sharp blade is essential to generate enough downward force to keep the shaving from levering up ahead of the blade tip to make a crack and grain tear. A sharp blade could be essential to an effective "mouth effect", to coin a term analogous to the Kato-Kawai "cap iron effect".