Recently a new fan-hinged shear rupture mechanism has been identified as a unique mechanism of dynamic shear ruptures. In the fan-mechanism, the shear rupture propagation is driven by a fan-shaped rupture head consisting of an echelon of intercrack (domino-like) blocks formed due to the consecutive creation of small tensile cracks in the rupture tip. The fan-structure propagates through the intact material as a wave and has a number of extraordinary features, one of which is extremely low shear resistance of the rupture head (below the frictional strength). Here we present a mathematical model elucidating the principles of this new mechanism. The model will support comprehensive studies of unique features of the discovered phenomenon.