I don't think this concept will come across as anything new to those on this forum but I thought I'd put some ideas down.
I've been investigating some of the origins of karate techniques, in particular the influence of Chinese Kung-Fu on karate.
It's often said that some of karate originates from Kung-fu or this or that kata originates from White Crane kung fu but really it's not entirely clear how one culture or the other influences karate, what exactly is it that comes from the external culture? Stances, ways of punching? Philosophy etc? I'm going to put the point that over and above the importation of entire kata, it's discrete "combative units" which are taken from one or another sources. A "combative unit" being the consecutive sequence of kata movements which gives rise to an application.
There has been a concept of kata as an almost stream of consciousness continuing sequence of individual combative movements, that is largely how they were taught historically in karate classes and how I understood them for a long time, but as a tool it doesn't hold water.
Patrick McCarthy has done an enormous amount of research on the origins of kata and came to the conclusion that they began as paired drills of techniques, attacks and defences. The defence part of which were assembled together into forms for better memorisation and time and space efficient practice. I now view this as a highly likely explanation and have some direct evidence for this sort of methodology for kata.
The key evidence for me is Iain's manji-uki application for which I consider the fit between the movements and the application to be far too good to be chance. One of the nice things about manji-uke is the highly characteristic body shape. It's very unusual and easy to identify making it a good probe. In fact I recall having seen exactly this shape in many martial arts films, used arbitrarily of course. In particular the manji-uke shape is definitely present in kung-fu and therefore will be present in Chinese kung-fu forms.
With the internet and youtube it has become easy to find examples of forms, indeed there are some channels with hundreds of videos of kung-fu forms. https://www.youtube.com/user/shemmati10yt being a notable example.
After having viewed many of the forms uploaded to youtube I hit upon an example of the manji-uke shape in one of the videos. In fact it's quite popular, but the particular one I'm referring to is this (tong bei quan), at 1 minute 15 seconds through to 3 minutes 10 seconds:
Approximately: manji-uke followed by rotating jump into shiko-dachi with downward strike, and then double punch.
Beyond the point that the Shaolin monks clearly have no better applications than those which have been historically present in karate (Despite seeming to have information that it's related to defence against a kick). Visually, the sequence has a remarkable similarity to the manji-uke sequence found in Passai/Bassai. It is almost identical visually with minor stylistic variations and the application of the sequence matches the application of the sequence in Bassai perfectly. i.e. The Bassai sequence was lifted completely and as a discrete "combative unit" from the kung-fu form (if not this specific one, then another very similar).
Overlapping and overloading
It's certainly possible for there to be overlapping applications, and double/multiple applications for sequences, but I don't think they were the primary use, they are happy coincidences and one of the side effects and benefits of the bunkai process if they're then tested and found useful.
Progression and variation
Progression in kata as demonstrated by Iain's analysis methodology whereby if X fails then progress to Y does *appear* to be a feature of the combative units... This is something I'm still looking at and I'm not totally clear on yet, the following sequences in the kung-fu form are sufficiently different from the Bassai ones that i'm not sure they match, there are similarities. If they don't then progression may only be a feature of kata or Bassai may have enhanced or expanded progression.
Size of a combative unit
How big is an application/combative unit then? Obviously it's impossible to generalise. The Bassai example is at least 3 distinct movements and possibly substantially larger if the follow on movements in the kung-fu forms also match.
So it's pretty clear then that karate kata and Kung-fu forms are both made up of discrete chained "combative units" representing applications, that can be extracted whole and used elsewhere. It's also clear that stylistic differences are relatively minor even between kung-fu and karate never mind between styles within the respective martial arts. The information is relatively intact.