Auto-Tune EFX pitch correction works by continuously tracking the pitch of the source and comparing it to the notes in a (customizable) scale
The scale note closest to the input pitch is identified as the target note. If the vocal's pitch exactly matches the target note, no correction is applied, but if your vocal's pitch varies from the target note, either sharp or flat, the output pitch is smoothly corrected to the target note
The Auto-Tune Vocal Effect is what is technically known as "pitch quantization", limiting each note to an exact pitch, stripping out any variation, as well as forcing instantaneous transitions between notes