The shock dispersal of particle shells or rings by radially divergent impulsive loads takes the form of coherent particle jets which have much larger dimensions from those associated with the constituent grains. In the present study, quasi-two-dimensional particle jetting is studied via both experiments based on a radial Hele-Shaw cell and numerical simulations using the discrete element method. Simulations agree well with the experiments in terms of the branched jet pattern and characteristic timescale for jet growth. Besides simulations reproduce the signature events defining the jet formation observed in experiments, specifically the initiation of incipient jets from the non-perturbed internal surface of ring, as well as the concurrent ramification and annihilation of jets. More importantly the particle-scale simulations reveal the physics underlying the jet inception. We found that the onset of particle jetting corresponds to the transition of the shock jammed band which homogeneously expands outwards to the unevenly spaced localized shear flows, or equivalently, unjamming of the dynamic jamming front. (C) 2018 Elsevier B.V. All rights reserved.