The twenty fourth of Teves is the Alter Rebbes yortzeit. The Alter Rebbe's father was a chosid of the Baal Shem Tov but the Baal Shem Tov told his father not to tell him that he is a chosid. He wanted that the Alter Rebbe should come to chassidism on his own. Also the Baal Shem Tov did not want that the Alter Rebbe Should see the Baal Shem Tov. He wanted that the Alter Rebbe should be mekushar to the Maagid of Mezritch.The Alter Rebbe was married already and he followed the path of avoda as taught by the Shaloh but at a certain point he felt that he was not satisfied with his service of G-d. At that time there were known to be two great centers of Torah, the Vilna Gaon in Vilna and the Maagid in Mezritch. It was also known that if you want to learn how to learn (nigla) you go to Vilna and if you want to learn how to serve the Aibishter you should go to Mezritch. The Alter Rebbe said that to learn he knows how to but to serve G-d he could improve on so he set out for Mezritch. He stayed there for a short time and he liked it but did not see anything there which should compel him to stay so he decided that he will go back. He was getting his horse and carriage ready to go. When he was there in Mezritch the Maagid gave his son Avraham the Malach to learn with Alter Rebbe. So now the Malach said to the Alter Rebbe "when you whip your horses you should whip them until they know that they are horses" and another version is "you should whip your horses until they cease to be horses". When the Alter Rebbe heard this he said that there is something very deep behind this and that he must go back and learn what it is.
The horses represent the body of the person or his animal soul and one must not let his animal soul take control of what the person must do or think but the body has to be made known that it is only a "horse" and to obey it's owner. This way the body can accomplish very great things but only when it knows who it's master is. The second higher level of service is when the body and animal soul itself are transformed and become G-dly themselves and desire nothing else but G-d.
So the Alter Rebbe made a u turn and went back to Mezritch and became the greatest disciple of the Maagid.