Why does a commutator reverse connections twice during each rotation of a DC generator?

A commutator reverses the connections in a DC generator twice during each rotation to ensure that the output voltage remains unidirectional.1

Because the voltage induced in a rotating coil is inherently alternating—meaning it changes polarity as the coil flux increases and decreases—the commutator acts as a mechanical switch. It reverses the connections at the two specific instants in each rotation when the rate of change of flux in the coil is zero; that is, when the enclosed magnetic flux is at its maximum (positive) or minimum (negative) value. By performing this reversal, the commutator effectively converts the alternating current generated within the armature into a unidirectional output at the stationary terminals.12