The use of this term has been debated among professional avalanche workers, but it is commonly used to refer to a situation where a strong layer of snow lies over a weak layer, thus "bridging" it.
This is common mechanics problem in some ways, often called the "Simply Supported Beam with a point load". However, in the snowpack this bridge can be any length in any direction along the surface. The distance between supports is generally unknown. The material properties of the bridge material are poorly understood and often vary significantly throughout the bridge. And depending on how many people there are and how large the "bridge" is there may be multiple point loads.
So when the term "bridging" is used it implies a weak layer underneath a stronger layer which is at least somewhat supportable. The question of whether the bridging is consistently strong enough to be safe to travel on is a difficult one.
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