Some guy from reddit, created this thread, I'm borrowing it because it seemed interesting:
Concentration of Death is something that was experienced by Hanma Baki in the Grappler Baki anime/manga series (Episode 4 of the Grappler Baki anime). It's described as an experience where concentration reaches a whole new level and your whole life's memories are experienced in the brief moment you first experience it. By the way, the experiencing of large portions of your life memories in an instant, due to near death experiences, is actually a real life phenomena called "life review". Anyway, Baki is able to see things in slow motion when he uses this concentration similar to how Ogata saw Takeda's and Lugh's battle in slow motion. Rittou No Kan means, 1/10th of a moment sense and it's likely something Kenichi acquired when he used Ryuusei Seikuken vs Sho Kano, given how Sho described Ryuusei Seikuken as raising the 5 senses to the limit, the same as what Miu did when she used Ritto No Kan.
Remember when Kenichi was facing death, due to a big rock falling above him in his match against Sho Kano? It's just like what Baki experienced, when he jumped off the cliff. They both had near-death experiences and that near-death moment with the rock is when Kenichi achieved Ryuusei Seikuken. Baki's near-death moment jumping off the cliff is when he awakened the Concentration of Death.