We explain the physical mechanism of ultra-fast flame accelerationin obstructed channels used in modern experiments on detonationtriggering. It is demonstrated that delayed burning between theobstacles creates a powerful jet-flow, driving the acceleration.This mechanism is much stronger than the classical Shelkinscenario of flame acceleration due to non-slip at the channelwalls. The mechanism under study isindependent of the Reynolds number, with turbulence playing only asupplementary role. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory isvalidated by extensive direct numerical simulations and comparisonto previous experiments.