Luke said to Obi-wan, "I can feel the good in him." What Luke is saying is that although Darth Vadar has done to an extreme yang position, ergo the dark side, he still could feel/see the "seed" of good or yin in him and by casting doubt by his actions Darth began to feel the shift. He had reached the end and as symbolized by the I Ching, that sixth line for Darth, ended thus taking its natural way toward its opposite, yin.
Darth becomes good again and stands beside Obi-wan and Yoda. It just goes to say that we all have choices and the I Ching provides us a means to choose which direction we travel. Think about this when viewing the above.
As for Luke, up to this moment, he vacillated between his yang and yin until this brought about equilibrium. He fought the dark and light side where his free will could have chosen either path but his greatness within chose the correct and centered path.
Isn't Jedi just another way to say, "Hunter/Warrior?" ;-)