Let g be a simple Lie algebra and W be the simple W-algebra, associated to g and to a
to a minimal nilpotent element in g. A level k is called collapsing if W is isomorphic to
its affine vertex subalgebra. Collapsing levels have been investigated jointly with Adamovic,
Kac, Moseneder Frajria and Perse, in connection with the representation theory of the simple
affine vertex algebra associated to g. We will discuss applications of this notion,
in particular to the semisimplicity of a certain remarkable category of representations.