We study the β-SiC(100) [Formula: see text] reversible phase transition, using first-principles molecular dynamics simulations to search for the ground state atomic structure as well as to investigate the dynamics of this surface. We find that this surface consists of weakly bonded asymmetric Si dimers that exhibit a complex atomic motion, associated with a surface soft phonon. This soft phonon is strongly coupled to the electrons in dangling bond states close to the Fermi level, explaining the observed insulator-metal transition. We identify the dynamical processes responsible for the phase transition and predict that this surface should undergo another reversible phase transition at low T.