The first plague with which Hashem struck Egypt was the plague of blood. When Hashem commands Moses regarding the plague, He says, by way of introduction, "Pharaoh's heart is heavy; he refused to send the people." The question arises: what does God want to tell Moses with this? Didn't He already tell him, "And I will harden Pharaoh's heart... and Pharaoh will not listen to you"? Moses already knows that Pharaoh's refusal to send the Children of Israel stems from the fact that Hashem hardened his heart. This hardening of the heart itself came as punishment for Pharaoh's rebellion against the Most High, for which reason it was decreed that he would oppose fulfilling God's will so that he would consequently receive his punishment. But since Moses already knew all this, how is it possible that Hashem motivates the plague of blood with "Pharaoh's heart is heavy"?
HEAVY IN AND OF IT
The key to understanding this topic lies in Rashi's commentary. He explains the term "heavy" as a descriptive noun, not a verb. In other words, Pharaoh's heart "is heavy" in and of itself, not (only) as a result of an action, because God hardened it.
Pharaoh's heart "was heavy" by nature, possessing a natural stubbornness that caused it to oppose the will of Hashem; (in addition to this, Hashem made it even heavier, to harden it beyond its stubborn nature).
BRAIN, HEART, LIVER
This characteristic of stubbornness of heart is called in Hasidic teachings "the klipah of Pharaoh."3 The letters of the Hebrew word Pharo also compose the word 'haoref,'4 meaning (the stubbornness) of the nape of the neck. This characteristic implies an irrational obstinacy that causes a person to oppose even the dictates of their reason and feelings.
This is also alluded to in the Hebrew word used here to say 'it is heavy' - kaved - which can also be translated as liver. It is known that a person's abilities are generally classified based on the three main organs: the brain (reason), the heart (feeling), and the liver (executive power).
The correct order is that first, the person thinks about the issue and arrives at some logical conclusion; then, they develop the corresponding feelings toward the issue in their heart; and finally, based on reason and the feelings behind the decision, they move on to the channels of practical execution. But when "kaved the heart of Pharaoh," that is, when the heart is transformed into the liver and executive obstinacy takes the place of reason and emotion, then we have stubbornness in the best sense of the word.
MOSES BEFORE PHARAOH
This obstinacy of the 'klipá of Pharo' can appear in anyone and hinder their service to the Creator. Faced with it, one must employ sacred 'obstinacy,' the acceptance of the Heavenly Yoke. The Jew must instill within himself the acceptance of God's Yoke, which is independent of logic and emotions, regardless of his mood or situation. He assumes the yoke of the Most High and fulfills His Will in every condition and circumstance.
This power was possessed by Moses. Beyond his wisdom and greatness, Moses was utterly subservient to God. When the matter concerned God's will, he set aside his understanding and emotions and surrendered completely to fulfilling God's will. This is why he, specifically, was able to overcome Pharaoh's stubbornness.
(Likutei Sichot, Volume 31, Page 28)
NOTES: 1.Shemot 3:14 2.7:3,4 3.See Ohr Torah 4.Likutei Torah 5.Zohar Part 2 153,2 6.BeShalach 16:7,8