  We present a new, fully anisotropic, criterion for formation of trapped
surfaces in vacuum. More precisely we provide conditions on null data,
concentrated in a neighborhood of a short null geodesic segment (possibly flat
everywhere else) whose future development contains a trapped surface. This
extends considerably the previous result of Christodoulou \cite{Chr:book} which
required instead a uniform condition along all null geodesic generators. To
obtain our result we combine Christodoulou's mechanism for the formation of a
trapped surface with a new deformation process which takes place along incoming
null hypersurfaces.
