Experiments on frictional stick-slip instability in brittle materials and natural observations show that friction falls towards zero in the head of shear ruptures propagating with extreme velocities (up to supershear levels). Although essential for understanding earthquakes, fracture mechanics and tribology the question of what physical processes determine how weakening occurs is still unclear. Here, using a mathematical model, we demonstrate that the extremely low friction can be caused by a fan-like fault structure formed on the basis of a tensile-cracking process observed in all extreme ruptures. The mathematical model visualises and describes the fan-structure as a mechanical system during rupture propagation. It explains some features observed in laboratory experiments.