Waki Gatame is an armlock from Judo, Hapkido and traditional Japanese Ju-jutsu systems, but it can also be used in on the ground BJJ!
It is one of the official 29 grappling techniques of Kodokan Judo. It is one of the nine joint techniques of the Kansetsu-waza list, one of the three grappling lists in Judo’s Katame-waza enumerating 29 grappling techniques. Falling directly to the mat while applying or attempting to apply the Waki gatame in competition is listed as an Hansoku-make (Grave Infringement) by the International Judo Federation.
This armlock is executed with Uke on their stomach, while tori has uke’s arm trapped with the elbow beneath tori’s armpit. Tori lifts uke’s hand, causing hyperextension of the elbow joint in a similar manner to Ude-Hishigi-Juji-Gatame.
It’s a very dangerous move as the opponent barely has time to tap to it before the arm gets broken…
Here Shinya Aoki applies Waki Gatame in MMA:
Here is how to do it and on the ground in BJJ: