The Kumon Dojo was the dojo that was run by Ryu Kumon's father. Very little is known about dojo, save that it was somewhere far from Furinkan and very run down. There were no students, its sensei, Mr. Kumon, had no idea how to attract students or any other job, and it was literally falling apart, with broken walls and a dilapadated roof. The only thing supporting it was a single central pillar.
Genma Saotome encountered this run down dojo either just before Ranma was born or very early on the training trip. Meeting Mr. Kumon, he felt sympathy and offered to help him, giving him a scroll detailing a series of techniques that he had created, the Yama-Sen Ken. Genma was privately thinking, as he would later admit, that Mr. Kumon would use the techniques to become a bandit. That way, he could fix up his dojo and make it more attractive to students, who would come to learn the Kumon style.
However, that was not to be the case. Mr. Kumon either was too noble to use the moves in such a way or perhaps never saw beyond the "human body as a house" metaphor. Whatever the reason, he devoted himself to mastering the Yama-Sen Ken in an effort to use them to attract students in and of themselves. However his use of the techniques against the dojo's structure, in particular the "Crushing Embrace of the Killer Grip" against the solitary pillar supporting the dojo, broke it and brought the dojo collapsing on top of him.
Ryu Kumon survived, but his father was mortally injured. With his last words, he gave the scroll of the Yama-Sen Ken to his son and begged him to both master it and to discover the other half of the style, the Umi-Sen Ken, which he could then use to rebuild the dojo. Genma somehow discovered the fate that had befallen the Kumon Dojo and believed that the Yama-Sen Ken had died with Mr. Kumon, not knowing that Ryu had survived. He decided to seal away the Umi-Sen Ken by sending it to Nodoka to get rid of so he would not know where she had disposed of it, but in a demonstration of frugality she kept it to make envelopes out of.
Coincidentally, Genma declared that both styles were to be sealed following the destruction of the Kumon Dojo. , but Genma Saotome declared both of the 'senken styles to be sealed techniques shortly after the destruction of the Kumon Dojo.
Ryu Kumon spent the next decade wandering Japan, mastering the Yama-Sen Ken and seeking out the scroll containing the Umi-Sen Ken. However, his dream was to come to nought when he was defeated by Ranma using the Umi-Sen Ken against him and the true, shameful nature of both styles was revealed to him.