The mechanisms of reorientation of individual DNA molecules undergoing Pulsed Field Gel Electrophoresis (PFGE) have been studied using T2 DNA molecules labeled with acridine orange and visualized with a fluorescence microscope. It is shown that molecules undergoing PFGE and conventional electrophoresis often get trapped in hook conformations (narrow U-shapes) that play an important role in determining the mobility of the molecules. It is found that the mechanisms of formation of hooks require the previous generation of a kink (in which parts of the molecule double up inside a pore). Computer simulation experiments are presented to clarify the role of hook and kink formation in the size-dependent separation observed in PFGE experiments.