For example, if the negate flag is false and the shift amount is zero, the change amount equals the current period, making the target period equal to twice the current period.
This would imply that if the sweep is positive and the shift is zero, then the sweep should cause the pitch to lower one octave each time the divider reaches zero. In Mesen however, if the shift amount is zero, it doesn't sweep at all. I found an old post by Blargg that says:
Does setting the sweep shift to 0 result in no sweep?When the channel's period is less than 8 or the result of the shifter is
greater than $7FF, the channel's DAC receives 0 and the sweep unit doesn't
change the channel's period. Otherwise, if the sweep unit is enabled and the
shift count is greater than 0, when the divider outputs a clock, the channel's
period in the third and fourth registers are updated with the result of the
shifter.