Imo, weak human leadership, lack of discipline not to be confused with physical punishment and the most importantly, all 4 paws must always remain in contact with the ground at all times.
One of Caesar's go to statements in his shows that I fervently believe is that he does not rehabilitate dogs, he rehabilitates owners. He teaches owners how to be a pack leader, how to maintain strong leadership and the difference between discipline and physical punishment. I believe what he says when working with extremely aggressive or introverted dogs- Dogs live in the present. Unlike humans they do not live in the past nor do they hold grudges. They move on without hesitation.
Possession is 9/10ths of ownership. By allowing your pup on your bed, it has possession and ownership. As the pack leaders everything belongs to you or another human and you can take it back at any given time and they must yield without objection because that is exactly the social hierarchy of the all dog pack. When alpha moves from point A to B any subordinates between those points will yield and move for the alpha. If a subordinate pack member has something alpha wants the subordinate will yield to alpha however alpha does not take it head on but rather approaches the subordinate from behind the front shoulder and nudges the other out of the way - just as Caesar does on the show.
Pack leadership is a language of very subtle nuances. rarely is it physically aggressive or violent. Learn that language and it will be possible to read your dogs body language and how to intervene with discipline before the dog is out of controll and nothing will work. Another one of Caesar's technique.