Well often what happens when people do hanging leg raises is that their hip flexors end up pulling their pevlis into anterior (forward) tilt. He's simply clarifying that the opposite needs to occur: you need to tilt your pelvis toward the back of your body.
Think of pulling your pelvis toward your rib cage. Or, lie down on your back, place your fingers under your lower back, and flatten your back against your fingers using your abdominal muscles. That's posterior pelvic tilt.
Sorry if that didn't make sense