We investigate dynamic fracture of heterogeneous materials experimentally by measuring displacement fields as a rupture propagates through a periodic array of obstacles of controlled fracture energy. Our measurements demonstrate the applicability of the classical equation of motion of cracks at a discontinuity of fracture energy: the crack speed jumps at the entrance and exit of an obstacle, as predicted by the crack-tip energy balance within the brittle fracture framework. The speed jump amplitude is governed by the fracture energy contrast and by the combination of rate-dependency of fracture energy and inertia of the medium, which allows the crack to cross a fracture energy discontinuity at constant energy release rate. This discontinuous dynamics and the rate-dependence cause higher effective toughness, which governs the coarse-grained behavior of these cracks.