the above diagram shows two examples. for the 5,6-dimethylcyclohexa-1,3-diene (top row of diagram), only a disrotatory mode would result in p-orbitals having the same symmetry as the homo of hexatriene. the two p-orbitals rotate in opposite directions. for the 3,4-dimethylcyclobutene (bottom row of diagram), only a conrotatory mode would result in p-orbitals having the same symmetry as the homo of butadiene. the p-oribtals rotate in the same direction.