The most striking features of the Raman spectrum of boron carbide under pressure are explained theoretically by computing the Raman tensor using density functional perturbation theory and the second-order response. While the observed pressure-induced changes in frequencies and intensities of all of the peaks above 450 cm−1 are convincingly explained by the vibrations of (B11C) icosahedra and C-B-C chains – that have been identified for long as the two main components of the atomic structure of pristine B4C –, the puzzling non-monotonic behavior of a broad Raman band at low frequency, whose intensity increases under pressure up to 44 GPa, decreases and then vanishes, was so far unexplained. We find that the behavior under pressure of both the frequency and intensity of this band turns out to be remarkably accounted for, in the calculations, by the activation of the chain bending mode in atypically flexured chains. We show that the flexion of the chain occurs at high pressure in presence of interstitial B atoms that, at ambient pressure, sit in the prolongation of standard C-B-C chains. We propose the ambient-P mode observed at 270 cm−1 as a fingerprint for the identification of both such B-C-B-C chain-defects in boron carbide and local deviations from the rhombohedral symmetry.