The first flyer stands on the platform with one hand on the bar while the second climbs onto their back, wrapping their legs around the other flyer's waist and their arms around the neck. Then the first flyer does a One-Handed Departure with the second flyer on their back. If lines are used, it is the second flyer who wears them.
This departure is used for a Piggy Back Catch (Trick ID: 058).
You can find out more about this trick in my books: