Propagation refers to the spreading of a fracture in the snowpack. Slab avalanches occur when a fracture propagates along a weak layer on a slope that is steep enough to avalanche. The fracture then propagates through the slab as it separates and slides away from the surrounding snowpack.
The farther these fractures propagate, the larger the resulting avalanche will be. For this reason, propagation propensity, or the tendency for a fracture to spread, is an important indication of the destructive potential of a slab. Thicker and/or harder slabs tend to propagate wider than thinner or softer slabs.
The extended column test and propagation saw test are ways to test the propagation propensity of a slab over a weak layer.