Something about the wagons Yosef sent him must have caught Yaakov's attention: "They recounted all that Yosef had said to them and he saw the wagons that Yosef had sent to transport him, and the spirit of their father Yaakov revived." What was so special about the wagons?
Chazal explain that Yaakov sensed a subtle allusion intended by Yosef through the wagons. The Hebrew word for wagon is "agala," which resembles the word "egla," a calf. Through the wagons, Yosef hinted at the halakha of "egla arufa." This law requires that upon the discovery of a murder victim with no evidence, the elders of the nearest city conduct a ceremony in which they break a calf's neck and declare their having no knowledge of negligence with regard to the victim. Chazal explain that this was the last halakha that Yaakov and Yosef studied together before the latter's abduction. This is why the wagons, which symbolized the "egla arufa," struck an emotional chord within Yaakov. The question, though, remains, what does this halakha have anything to do with Yosef's bringing Yaakov to Egypt?
Rav Shimon Shkop suggested that upon catching Yosef's hint, Yaakov realized that his son had incorporated the lessons he had learned from his father about leadership. Already during Yosef's childhood, Yaakov must have sensed Yosef's future as a leader. He therefore taught him the laws of "egla arufa," which point to the primary element of leadership - responsibility. When a murder victim is found, the nearest authoritative body must assume the responsibility of investigating the murder. As Chazal explain, they investigate not only the murder itself, but the climate in which it occurred. They must explore the possibility that perhaps the victim left the city without adequate provisions, which may have indirectly led to his death. The onus falls upon the elder statesmen of the city to identify and correct the social ills prevalent among their constituency. All this Yaakov imparted to his son, Yosef, who was destined to assume the responsibility of leadership.
Thus, Yosef wanted to allude to his father that he never forgot the critical lesson he learned. He had said to his brothers, "You must tell my father everything about my high station in Egypt and all you have seen…" Through the wagons, however, Yosef assured his father that his "high station" was not misused. To the contrary, "God has sent me ahead of you to ensure your survival on earth, and to save your lives in extraordinary deliverance." Yosef utilized his royalty for the sake of helping and saving others.
Thus, when Yaakov realized that Yosef was a leader in the spirit of the "egla arufa," "the spirit of their father Yaakov revived."