As a new type of noncovalent interactions, pnicogen bond between a VA group element (N, P, and As) and an electron donor (Lewis base) has grabbed attention in recent several years. Here we employ the block-localized wave function (BLW) based energy decomposition scheme to probe the bonding nature in a series of substituted phosphines XnPH3-n complexed with ammonia. As the BLW method can derive the optimal monomer orbitals in a complex with the electron transfer among monomers quenched, we can effectively examine the HOMO-LUMO interaction in the pnicogen bonding systems. Among various energy components, electron transfer energy together with the polarization energy dominates the pnicogen bonding energy. Although usually it is assumed that the electron transfer from ammonia to substituted phosphines occurs in the form of n -> sigma* (XP) hyperconjugative interaction, we identify a kind of new pathway when X = NO2 and CN, i.e., n -> d pi*, which results from the interaction between the pi orbital of cyano or nitro substituent and d orbitals on P. But still this picture of electron transfer using a single pair of orbitals is greatly simplified, as the electron density difference (EDD) maps corresponding to the overall electron transfer processes show the accumulation of electron density on the P side opposite to the X-P bond, with insignificant or even neglible gain of electron density on the substituent group side. Thus, the EDD maps tend to support the concept of sigma-hole in pnicogen bonds.