This is a good strip to demonstrate the change in the appearance of a T wave when a premature P wave occurs on the preceding T wave.  The PACs found the atria ready to depolarize and produced a P wave that landed on top of the preceding T wave, making it appear taller than the others.  The PACs also reset the sinus node, causing a slight delay before the next sinus discharge.  The PACs occurred while the ventricles were still refractory, so no QRS complexes followed.