Naifuanchin is the grappling kata. Its name translates as 'internal divided conflict' — the same inner division described in Zen & Karate, written into movement. It holds the same status in the Shorin and Shuri-te systems that Sanchin holds in Naha-te; the presence of both kata across the old lines suggests a single source behind the art.
Naihanchi in Shorin-ryu and Isshin-ryu, part 1 only in Wado-Ryu, and Tekki 1–3 in Shotokan. The best-preserved version studied in Shizenryu was identified with the help of Shoshin Nagamine's The Essence of Okinawan Karate.
Naifuanchin is the summit of the kata curriculum. It first appears at Shodan (Section 1, in the Mon stage-A material) and is a standing kata requirement at Nidan and Sandan, where the NFC grips are studied — sword grip and scabbard grip, single hand. Those same grips are foreshadowed much earlier: Kumite 11 and 12 at 3rd Kyu are built on the crossed-hand sword and scabbard grips with their takedowns. It is also the final stage of the pushing-hands progression: Naifuanchin grappling.