"This is when a sheet of dense snow is formed by the wind driving it in such a way it forms a hard slab on an area of existing snow. But the problem arises when it overlies a weak layer snow which then fails," he said.
"The weight of a person can cause a localised collapse. If I stood on snow that is 80cm deep I can affect the layers that are down there. When you have a hard wind slab you send a tremor which develops the snow pack which forms a crack."
He said that in this case it led to an avalanche across 400 metres.
Mr Diggins warned there would be deep instabilities in the snow pack this weekend
"We would urge people to look at our avalanche reports, particularly to establish where the hazardous places are and be mindful of them when they are planning their routes. Look to the safer area. It is the steeper convex slopes that are most at risk," he added.