When the fracture of a lower snow layer causes an upper layer to fall. Also called a whumpf, this is an obvious sign of instability.
Collapsing snow (sometimes mistakenly called "settlement") is when the snowpack collapses under you with a loud "whoomph." (Actually, whoomph has been adopted as a technical term to describe collapsing snow. Sounds funny but it's a great term.) Whoomphing is the sound of Mother Nature screaming in your ear that the snowpack is unstable and if you got a similar collapse on a slope that was steep enough to slide it wouldn't hesitate to do so. Collapsing snow occurs when your weight is enough to break the camel's back and catastrophically collapse a buried weak layer, most commonly faceted snow or surface hoar. Collapsing snow on a flat valley bottom can easily trigger avalanches on steeper slopes above and sometimes collapses can propagate very long distances and trigger avalanches on more distant steep slopes. Not surprisingly, collapsing snow means that the snow is extremely unstable. The weak layer is already holding up the weight of a significant amount of snow and just the wimpy addition of your weight can collapse all the snow in sometimes a very large area and can sometimes propagate long distances. Collapsing snow is an obvious clue (this is repetitive with same phrase used above) that you need to stay off of and out from underneath avalanche terrain.