The Naihanchi katas form the basis for much of the shuri-te karate, but other than the fact that they were often the first katas taught to students, we really know very little about them. They are typically taught as the first or beginner katas.
It is theorized that:
- The three katas were originally a single form and this seems reasonable.
- It is based on an earlier Chinese kung-fu form, which again is reasonable given the name, but we don't know what that form is or if it's still practised.
- The katas were designed for combat on the raised ridges between paddy fields, on boats, against walls etc. While the Naihanchi embusen is unique among Okinawan katas, these reasons are probably apocryphal.
Naihanchi is a kata that suffers a fair bit from information corruption and "drift". Every school of karate based on shuri-te has it's own variant, and they all differ slightly. Some variants have clearly been modified more than others to include additional applications beyond the drill sequences. In particular the stance varies. In some styles they use the naihanchi stance, other styles use kiba-dachi (horse stance), and yet others use heiko-dachi (natural stance) and others shiko-dachi. The most common appears to be kiba-dachi. Very clearly then, the specific stance doesn't matter, as long as it exhibits similar general principles. Some styles however have modified their kata to the point that common oyo bunkai wouldn't work as normally performed.