Advantages of the analysis of the topology of the electron localization function (ELF) in the characterization of the chemical bonding in solids are illustrated in the study of the zinc blende -> rock salt transformation in BeO. The 4-fold to 6-fold coordination change is described as a two-step process: first, a catastrophic-like emergence of two new Be-O bonds reveals the onset of the rock salt structure, and second, the new interactions gradationally evolve to achieve the bonding network of the high-pressure phase. The increase in coordination, the volume collapse and the enhancement in the bulk modulus across the transition pathway are qualitatively and quantitatively traced back to the oxygen's valence shell. Although several ELF indexes point toward the expected greater bond polarity in the B3 than in the B1 structure, it can be concluded that there is no substantial modification in the nature of the crystal interactions induced by the phase transformation.