Mules are not physically capable in keeping an unnatural head carriage - as  pictured to the left.  Their poll area is thicker and stronger as compared to the horse and their throat latch/jaw area is much heavier - this trait came from the donkey side of the family.  For the mule to physically tuck their head and nose requires light and responsive hands over time gradually.  It is extremely uncomfortable for the mule to carry his head in this manner naturally.  Heavy hands or non-responsive hands encourages the mule to pull against your reins.  The mule's esophagus allows airflow (outlined in blue in pic) that enables the mule's very survival - to breath.  That being said, you should have a better understanding as to why mules can easily develop into runaways when they are being handled/trained by a forceful or unknowledgable rider/trainer with heavy hands.  No matter what bit or hackamore you use on your mule - to be successful in your training, you have to be able to "feel" your mule's response and be able to give back at that moment to enable him to be comfortable in the bridle and not fight against you.  This is direct communication coming from your hands to your mule's mouth/jaw. Establishing a light contact with your mule in the bridle is a realistic and reachable goal. Train your mule to do what he is physically able to do naturally and your working/riding relationship will improve.  ~Cindy K. Roberts