We start talking about it at the 36:20 mark...
Deep Magic (ch. 13):
“Have you forgotten the Deep Magic?” asked the Witch.
“Let us say I have forgotten it,” answered Aslan gravely. “Tell us of this Deep Magic.”
“Tell you?” said the Witch, her voice growing suddenly shriller. “Tell you what is written on that very Table of Stone which stands beside us? Tell you what is written in letters deep as a spear is long on the firestones on the Secret Hill? Tell you what is engraved on the sceptre of the Emperor-beyond-the-Sea? You at least know the Magic which the Emperor put into Narnia at the very beginning. You know that every traitor belongs to me as my lawful prey and that for every treachery I have a right to a kill.”
“And so,” continued the Witch, “that human creature is mine. His life is forfeit to me. His blood is my property.”
"[Aslan] knows that unless I have blood as the Law says all Narnia will be overturned and perish in fire and water.”
Deeper Magic (ch. 15)
“It means,” said Aslan, “that though the Witch knew the Deep Magic, there is a magic deeper still which she did not know: Her knowledge goes back only to the dawn of time. But if she could have looked a little further back, into the stillness and the darkness before Time dawned, she would have read there a different incantation. She would have known that when a willing victim who had committed no treachery was killed in a traitor’s stead, the Table would crack and Death itself would start working backwards. [...]"
Aslan: If the Witch knew the true meaning of sacrifice, she might've interpreted the Deep Magic differently: That when a willing victim who has committed no treachery is killed in a traitor's stead, the Stone Table will crack and even death itself will turn backwards.