What is Kosen Judo?

Kosen Judo is a subcategory of Judo specializing in ne-waza (ground fighting). Judo is also known as Kodokan Judo or regular Judo. According to El Dorado Hills Jiu Jitsu, "One of the most fascinating influences on Brazilian Jiu Jitsu (BJJ) is Kosen Judo, a specialized form of judo with a strong emphasis on ground fighting, or ne-waza."

Kosen Judo "originated in Japan in the early 20th century as a subset of traditional judo. While judo, founded by Jigoro Kano, is known for its dynamic throws and stand-up grappling (tachi-waza), Kosen Judo placed much greater emphasis on ground techniques, including pins, joint locks, and strangles. The style developed largely within Japan's university system, where teams competed under special rules that favored longer periods of groundwork, unlike traditional judo competitions."

As to the impact of Kosen Judo, it can be describe as such, "Brazilian Jiu Jitsu owes much of its foundation to judo, and especially to Kosen-style ne-waza. When Mitsuyo Maeda, a Kodokan judoka, arrived in Brazil in the early 20th century, he passed on his grappling knowledge to the Gracie family and others. Maeda's teachings included many of the ground-fighting techniques that had been refined in Kosen Judo, which eventually evolved into the distinct style we know as BJJ today."

For more general information on Kosen Judo, please see the wikipedia page.