The Amakakeru Ryuu no Hirameki
Kenshin knew that there was something he lacked, both as a swordsman and as a human being. It's why he left Kaoru in Tokyo and went to Kyoto alone. I think that he had no idea whether he could survive a confrontation with Shishio and come out of it as Kenshin, and not a raving murderous Battousai or something. When he reached Kyoto, the first thing he did was seek out a replacement for his sakabatou, broken in his confrontation with Seta Soujirou (Shishio's right-hand man). The next thing was to return to the master of the Hiten Mitsurugi Ryu, Hiko Seijuurou, and learn the succession techniques of his school.
Learning the Amakakeru Ryuu no Hirameki is nothing like learning normal techniques. During the time that Kenshin had spent being an assassin and wandering all over Japan, his master had been keeping track of him, and immediately knew that his "stupid pupil" was in no way prepared to inherit the techniques. Kenshin had left his training as a child, without fully understanding the principles of the Hiten Mitsurugi Ryu. This is clear when he declares that he would willingly give up his life fo the sake of protecting people. He was prepared to throw everything away. Hiko thought this was extremely foolish. However, since the succession test for the Hiten Mitsurugi Ryu meant the death of either master or student, Hiko decided to give him the test anyway, as it would be the best way to know if Kenshin was ready or not. After trying to drive the essence of Hiten Mitsurugi into Kenshin's head (and failing spectacularly), the test was on, and there would be no turning back.
However, just before the day of the test, Kaoru turned up at Hiko's doorstep, reminding Kenshin of what he had tried to leave behind. With thoughts of his friends, of promises he had made, Kenshin realized that he did not want to die. Once he found his will to live, he was able to figure out how to do the Amakakeru Ryuu no Hirameki by instinct, the way it was meant to be learned.
That the Amakakeru Ryu no Hirameki requires simply the will to live seems simplistic, but I think it makes a good point. If you're willing to die, then what's the point in fighting? It's rather sad that Kenshin did not see this right away, considering how badly he wanted to believe in the "sword that protects life." Dying would leave the people he wanted to protect sans a protector (whether they need protecting or not doesn't matter), but laying down his life for a cause wasn't going to help anyone. This was an important turning point in his life, when he realized that "Himura Kenshin" had some worth and realized that the people who cared about him would want him to return. This will to live compensated for Kenshin's unwillingness to kill, allowing him to be the equal of Hitokiri Battousai in strength.
However, in a way, this also made the Jinchuu arc which follows even more heartbreaking.