The x descent follows the c wave, and occurs as the AV valve moves downward with contraction of the right ventricle. It takes place at the same time as S2 and just after the T wave.
The v wave is the last in the sequence, and results from atrial filling during atrial diastole as the ventricles are beginning to relax.
Finally, the y descent occurs when atrial blood begins flowing into the ventricle. The cycle then repeats itself.