It's described (and matches my memory) of producing a "pulsing direct current". The commutator, a physical switch, maintains the polarity of current by breaking and switching between generator coils.
If I'm remembering and understanding correctly: the commutator swaps the + and - ends of the energised coil. So you're not actually zeroing out the inverted end of the wave, you're swapping its connections to the circuit. Where an AC generator coil applies a positive and negative voltage to a single lead, the DC generator swaps leads as the current reverses.
wikipedia seems to agree with you, if i read this right.
> A dynamo is an electrical generator that produces direct current with the use of a commutator.
>> A commutator is a moving part of a rotary electrical switch [...] that periodically reverses the current direction between the rotor and the external circuit
https://en.wikipedia.org/wiki/Dynamo
It's described (and matches my memory) of producing a "pulsing direct current". The commutator, a physical switch, maintains the polarity of current by breaking and switching between generator coils.