"Because there are voltage K gates open as well as the K leak channels the K⁺ permeability of the membrane is higher than normal, thus the membrane potential moves closer than normal to the K⁺ equilibrium potential. This results in a momentary undershoot of the membrane potential, before it returns to normal resting potential when the voltage sensitive K⁺ gates close."

courtesy of Robert H. Parsons, Ph.D., Rensselaer Polytechnic Institute, used with permission