The short answer to the title question is that it acts as an electrostatic bouncer that shoves the charge flow from flavin toward the DNA lesion that photolyase repairs. This explanation is provided by an explicit time-dependent quantum mechanical approach, which is Used to investigate the electron transfer process that triggers the repair mechanism. The transfer occurs from the flavin photolyase cofactor to the cyclobutane ring of DNA, previously formed by light-induced cycloaddition of adjacent pyrimidine bases. The electron wave function dynamics accurately accounts for the previously proposed mechanism of transfer via the terminal methyl group of-the flavin moiety present in the catalytic electron-donor cofactor, FADH(-), which also contains adenine. This latter moiety, which has Often been assumed to be present Mainly for structural reasons, instantaneously modifies the interaction between acceptor and donor by a variation of the electrostatic interactions so that the presence of its local atomic charges is necessary to trigger the transfer. Ill principle, knowledge of the details of the electron transfer dynamics and of the important role of polarization effects call be exploited to improve the efficiency of the repair mechanism in artificial systems.