What types of Military Discipline kept the Mongol army at its best?

The Mongol army was considered as the most disciplined army of their time. A structure was formed that had obedience as the ultimate criteria. They executed the order given to them by their superiors without questioning. The delegates of the Mongol army were miostly chooses based on their blood connection with the Khan family or through their greed deeds in the field of war. There was no provision of being transferred from one unit to another. The leaders of each group had the freedom to execute the orders in any way they deemed fit. Kheshig was the ultimate leader of all the groups present.