What is/was Kosen Judo?
Kosen judo was a refinement of Kodokan Judo that was developed and flourished at the Kōtō senmon gakkō technical colleges in Japan in the first half of the twentieth century. Kosen judo’s competition rules allowed for greater emphasis of ne-waza ( ground techniques) than in mainstream judo and it is sometimes regarded as a distinct style of judo. Today, the term “Kosen judo” is frequently used to refer to the competition ruleset associated with it that allows for extended ne-waza. Such competition rules are still used in the shichitei jūdō / nanatei jūdō competitions held annually between the seven former Imperial universities.
Differently to modern Judo rules leglocks were allowed (Leglocks started being prohibited by Kodokan rules in 1914 in shiai and randori as well. By 1925 all joint-locks except elbow locks were totally prohibited together with neck cranks. Kosen rules being the Kodokan rules derivative did not allow leglocks absolutely).
The matches had no time limit and were usually contested on a mat 20×20 meters in total size. A starting zone 8×8 meters was marked on the mat as well as a danger zone which ended at 16×16 Meters.
There has been a limited resurgence in interest in Kosen judo in recent years due to its similarities with Brazilian jiu jitsu but it is still nowhere as popular as modern Kodokan Judo.
How is Kosen Judo different or similar to Brazilian Jiu-Jitsu?
If you look at the Kosen Judo video below you can see many moves and flows that are different from modern Judo ground work (which is much more focused on pinning and more static). These videos were made in the 1970’s by 6 older judokas , among them (in the tapes) was the famous Masahiko Kimura himself (who beat Helio Gracie). When you look at the techniques displayed in the video, it looks very similar to Gracie Jiu-Jitsu (which is more focused on the basics than modern sport BJJ). Kosen Judo players also pulled guard in competition. Kosen had a pinning rule. Strategy is different than BJJ. There was more more turtling than BJJ etc.
Maeda, who taught Carlos Gracie old style Judo (Jiu-Jitsu) but not necessarily Kosen Judo, had learned some catch wrestling in the USA on his way to Brazil, so his style had some catch wrestling elements. You can say that Gracie Jiu-Jitsu had some catch wrestling to it, that Kosen Judo never had.
Modern sport Jiu-Jitsu has all the newer innovation such as 50/50, berimbolo, De la riva guard, reverse de la riva guard, kiss of the dragon, lapel guard, worm guard, inverted guard, tornado guard etc.. There are some positions and techniques that both styles share (The spider guard for example) but the general form of grappling is more a distinct flow instead of just a collection of moves. To give you an example, look at the way Keenan Cornelius, Miyao brothers or Mendes brothers roll and then look at the Kosen Judo players in the video below. It looks much more similar to Gracie Jiu-Jitsu than to modern sport BJJ.
In short, Kosen Judo and Brazilian (Gracie) Jiu-Jitsu are similar but are still very different. Most of the techniques are the same, the competition rules are different. Kosen Judo has faded due to the popularity of modern Kodokan Judo (more focused on throws), while BJJ is fastly growing and always evolving with new techniques and flows.
Check out this great video explained and demonstrated by the legendary Masahiko Kimura.
During the volume, Kimura and his student demonstrate ne-waza techniques which include: Joint locks, escapes, reversals, combinations and much more.
The video is in Japanese with no sub titles but the demonstrations from Kimura are easily followed visually, with the content more than compensating for this:
Interesting documentary on Kosen Judo, with english subtitles: