Click the structures and reaction arrows in sequence to view the 3D models and animations respectively
Norcaradiene ring opening is a type of electrocyclic reaction.
In a electrocyclic rection, one sigma bond is either broken or formed. Electrocyclic reactions react in either a disrotatary or conrotatary fashion. This ring opening is a disrotary reaction, see this page for more information.
O. A. McNamara and A. R. Maguire, Tetrahedron, 2011, 67, 9–40.