The change partial control is a slider-like control that works with VoiceOver just like a slider. After you select this control, you can swipe up or down anywhere on the screen to increment or decrement the selected partial by one.
Swiping downward will select lower partials until the lowest possible partial is reached, which is the Fundamental. Swiping upward will select higher partials until the highest possible partial is reached for the current note. The highest possible partial is different for each note. Lower notes allow much higher partials than higher notes. As the partial is changed, VoiceOver will speak the new partial selection without the need for the Current Partial display field to be selected.