Dry snow slab avalanche formation starts with failure in the weak layer underlying the slab. The stress has to locally exceed the strength that strongly depends on the strain rate. Providing the deformation energy released is sufficient, a shear fracture might propagate and eventually slab release takes place. Whereas field measurements suggest that skier triggering is common without hitting a deficit zone, it seems plausible that in the case of natural release only with the presence of imperfections the critical strain rate and strain can be reached to initiate the brittle fracture in the slab failure process. However, little is known about the type and size of these imperfections, and in particular about their lifespan. Model calculations give a wide range of values on the size, typically between 0.1 to 10 m. It seems unlikely that a deficit zone will survive for weeks (and wait for a skier), but rather will heal pretty soon within hours. Deficit zones are supposed to be a transient phenomenon. This might partly explain why there is hardly any direct evidence for imperfections from field studies e.g. on the spatial variability of the snow cover stability. Both, natural release and artificial triggering by skiers of dry snow slab avalanches are revisited.