The trans-pyramidalization distortion is taken as an example. The frontier molecular orbitals of the undistorted alkene possessing D symmetry have symmetries a (HOMO-1), b (HOMO), b (LUMO), and b (LUMO+1). The symmetry of the trans-pyramidalization vibration is b. A triple product of ground state, vibrational mode, and excited state that can be taken is b (HOMO) x b (trans-pyramidalizing vibrational mode) x b (LUMO+1) = a. Since a is the totally symmetric representation, the b and b molecular orbitals participate in an allowed interaction through the trans-pyramidalizing vibrational mode. The molecule will distort in a trans-pyramidal fashion (into C symmetry) in order to enable this interaction, which produces a more stabilized HOMO and more destabilized LUMO.
This treatment can be repeated for all other combinations of HOMO-1, HOMO and LUMO, LUMO+1. Notably, it is found that the HOMO and LUMO are symmetry-disallowed to mix.