We propose a physical mechanism that explains how "polarization jets" (PJ) or "subauroral ion drifts" (SAID) are formed in the course of a substorm and how they evolve. A PJ/SAID is considered to be the ionospheric signature of an inward moving injected plasma front. The flow shear that exists across this interface when it arrives in the vicinity of the plasmapause is responsible for the generation of intense electric fields in the premidnight sector, where PJ/SAID are observed. Quantitative simulation of this mechanism accounts for PJ/SAID width and peak drift velocity. The mechanism explains why PJ/SAID are observed poleward of or in the vicinity of the plasmapause. The inward traveling time of the injected plasma agrees with the delay between substorm onset and the apparition of PJ/SAID; the evolution of the ionospheric signature is consistent with observations as well.