We propose a Rapini-Papoular type surface anchoring energy through spherical harmonic expansion. The second order approximation yields naturally a two-coupling-constant energy in homeotropic alignment. Using this anchoring energy, we analyze the homeotropic to twisted planar (HTP) transition. The threshold and saturation properties of the HIT effect are analytically derived, in the framework of the continuum approach. The results demonstrate clearly that the HTP transition is the reversal of the twisted nematic effect.