differential amplifier and ac imput
In AC analysis, DC or common mode (CM) = 0 because you ignore it for analysis purposes since you normalize everything around the bias point. The common mode or DC is still there in reality and the AC is riding on top of the CM voltage. So it's not actually flowing backwards; It's still flowing forwards but in a smaller amount, as long as |-Vd/2| &lt; Vcm.

So in AC analysis you're looking at deviations/changes from the bias value, values that are relative to the bias, not the absolute values.