The first Kosen Judo competition ever took place in 1898. Kosen Judo utilizes the same uniform, techniques, and fighting area as judo, but with a different ruleset. The rules lean towards a much heavier emphasis on ne-waza, even allowing athletes to "pull guard" as in Brazilian Jiu-Jitsu (BJJ). There is discussion as to whether BJJ actually originated from Kodokan Judo, or Kosen Judo, as it is so closely related to the latter. What we do know is that Mitsuyo Maeda (a student of Jigoro Kano) brought Judo to Brazil in 1914. Maybe the most famous practitioner of this style of Judo was Masahiko Kimura. In his heyday, he was widely considered the greatest judoka of all time, famously breaking the arm of of Helio Gracie in a match in Brazil, and later coaching Canadian Doug Rogers to win an Olympic Silver Medal in the 1964 Tokyo Games.
What it means in our dojo, where any student has the possibility of coming to judo every day, is that we set Monday nights aside to focus strictly on ne-waza. This allows students to explore techniques they normally wouldn't get as much of a chance to work on, and gives students a break from ukemi (breakfalls).