It's the period by which the cell attempts to return the membrane back to the resting potential. Now, because at this particular point, let's say at this point, our potassium, the membrane of the cell, is more permeable to potassium than normal. That means it will go slightly below our resting potential. And this is known as the hyperpolarization period. And this is shown by this period here. Now, when we have the hyperpolarization period, our sodium, potassium, Atpace pump will begin to pump.