From a chin up position, bring your legs up through your arms. Nominally, your legs would be more or less parallel to the ground with you upside down. It would look something like an upside down L sit, with the back of your calves against the pull up bar. This is your starting position. Now, keeping your legs straight and your shoulders engaged, continue the rotation until you are in the bottom of the German hang. Then, using strength from your chest, shoulders, back, core, and arms, pull yourself back until the back of your calves touch the bar again. Think about keeping your knees locked out throughout the movement. It may require more hamstring flexibility, but I think this is a case where keeping your legs straight is more about focusing your intent than a flexibility lomitation.