Waki gatame is an arm lock using the arm pit as a lever.
Judo is made up of throwing (nage), grappling (katame or ‘newaza’) and striking (atemi) techniques. The grappling techniques (newaza) are split into pinning (osaekomi), choking (shime) and joint locks (kansetsu).
Brazilian Jiu-Jitsu (BJJ) is historically related to Judo’s newaza techniques. The newaza techniques were brought to Brazil by Mitsuyo Maeda, who trained and received his black belt at the Kodokan (the headquarters of Judo). For this reason, many of the techniques in Judo’s newaza and BJJ are the same or similar.
See a full demonstration of different kansetsu-waza (joint locking techniques) at: https://www.youtube.com/watch?v=WNI7AJr1ViA