As you know the U wave in the ecg is seldom seen especially in leads V2 and V3 physiologically especially during bradycardia.
And it is seen pathologically in Hypokalemia.
So how is it generated?
If you are thinking due to repolarization of papillary muscles, let me tell you that this hypothesis is now obsolete.
There are three trending theories at present trying to explain it:
1. Delayed repolarization of the subendocardial Purkinje fibres.
2. Prolonged repolarization of the midmyocardium ( M cells)
3. After-potentials resulting from mechanical forces in the ventricular wall.