We already have a hint to the answer from the story of Yaakov's shepherding of the sheep. On the one hand, Yaakov played the system and worked the loopholes, positioning the striped posts in front of the sheep at the time of copulation so that striped and speckled sheep would be conceived. However, such behavior, while not totally yasher, was still not deceitful, and was still playing by the rules. While less than ideal, such behavior is often the only way a person can survive when he is unprotected and in an inhospitable and foreign land. Such hostile environments have often been the reality of Jews in galut. We as a people have had to learn to survive and adapt, and we have excelled in this ability, learning how to play the system while keeping to its rules, no matter how unjust they may be.
They key in these situations has always been to do what we need to in order to survive, but do so while keeping one's integrity. And Yaakov, in his watching of the sheep, in no way "played the system" - he was the model of integrity and hard, honest work: "That which was torn of beasts I brought unto thee; I bore the loss of it; of my hand did you require it, whether stolen by day or stolen by night. Thus I was: in the day the drought consumed me, and the frost by night; and my sleep fled from my eyes." (Breishit 32:39- 40).
So this "playing the system" was a necessary, adaptive mode for galut, but it was not ideal. And Yaakov is alerted to this, I believe, in the dream of the angel which he relates to Rachel and Leah. In that dream, the angel informs Yaakov that it was only because of God's help - and not because of Yaakov's scheme - that the ewes bore speckled lambs. The scheming was not what led to success, it was God's watching over him that did so.
And now Yaakov comes to encounter Esav. A hard and powerful enemy, but one that he is meeting head-on and in the land of Canaan, not as a guest in another land. Will he be the Yaakov of twenty years ago, and engage in real deceit? Will he be the Yaakov of the house of Lavan, and try to secretly work the loopholes? Or will he confront the challenge directly?
The answer is not necessarily a foregone conclusion. Some people are never able to readjust to new situations and to abandon their old paradigms. As a case in point, we find that even today, some of our fellow Jews still approach the government, even the democratic government of the United States, as if they were living in the old oppressive regimes of Eastern Europe. When asked by certain yeshivot if they could deceive the United States government to get more funding for programs than they deserved, Rav Moshe Feinstein (HM 2:30) spends the majority of his response explaining that we live in a country that is not Tsarist Russia, and that in a country that protects its citizens, modes of deceit and trickery learned in previous generations, which were always problematic, are absolutely unethical and completely inconceivable.
Yaakov Aveinu, for his part, was able to learn these lessons on his own, and the path that he adopts is the straight and narrow one. While he prepares a gift, this is an act of appeasement, not of trickery. And he says so directly to Esav: "[It is] to find favor in the eyes of my master." When Esav tries to maintain a presence in Canaan, Yaakov politely but firmly begs off. There is no trickery - everyone knows the purpose of the gift, the meaning of Esav's accepting it (an accepting of Yaakov's apology) and the meaning of Yaakov's begging off of Esav's accompaniment and Esav's acquiescence to this (a waiving of Esav's rights to the land of Canaan). There is sophistication, intelligence and skill in this approach, but no trickery. And Yaakov has also not forgotten the angel's message. He knows that whatever he does, his success relies on God's help. Thus, before this momentous event he prays to God - the first time Yaakov talks directly to God - to beseech God for help and protection.
Yaakov has learned that to achieve his goals he need not use deceit, and he need not make use of loopholes. He can achieve his goals by confronting his challenges head-on, by using intelligence and skill, and by relying on God. This may be a harder path, it may be a riskier path, but it is the correct path. And to fully realize it, he must confront his old self and reject the Yaakov of the past. And thus, on the night before the encounter - "And Yaakov was left alone." He was left alone with himself. He had to look himself in the mirror and confront who he was and who he wanted to be. "And a man wrestled with him" - he had to struggle with his old self, and to reject who he was for who he could become. And when his old self saw that it could no longer define the new Yaakov, it engaged in its old trickery - "And he saw that he could not prevail against him" - when he - the man, the old Yaakov - saw that he could not achieve his ends through honest means, "he touched the curve of the thigh" - he used a dirty trick, touching the curve of the thigh, comparable to the curve of the heel, the ekev, the crooked path that is the old Yaakov. But the new Yaakov would not lower himself to this level. He refused to fight dirty, to give in to the old Yaakov, and he remained upright. And as such, he merited a new name: Yisrael. No longer Yaakov - the bent heel, but Yashar-El: Yashar, the straight one, El - who connects to God.
Yaakov became Yisrael, became the one who recognized that we must always remain straight and upright and rely on God, and that in this way - and not through self-reliance on trickery and deceit - will we achieve our goals. The path may be harder, we may have to sacrifice something as a result, we may come out limping a little in the end, but our integrity will be intact, and we will be better for it. That is why Bnei Yisrael - the descendants of Yisrael, not of Yaakov - will not eat the gid hanesheh that is on the thigh - that we reject the representation of the old Yaakov, and together with it give up some small degree of pleasure. This is a tiny sacrifice to make for being yashar and being with God, and for our meriting to be the true descendants of Yisrael, worthy of being called Bnei Yisrael.