In Brazilian jiu-jitsu, practitioners are used to defending front triangles. These triangles are the one usually set up from a guard position, closed half guard or open guard. There are numerous setups for the front triangle. You can set it up usually from a spider guard position from a lasso guard etc.
Another common triangle seen in Brazilian jiu-jitsu especially in the past few years, is the back triangle which is usually set up when you have your opponent’s back and you trap an arm and then lift the other arm, falling to the side to trap a triangle. This triangle is very difficult to get out of.
A few years ago, Braulio Estima shocked the world When he was able to submit many world-class opponents at ADCC 2009 with a reverse triangle set up from an open guard. Victims of this reverse triangle included Andre Galvao and Xande Ribeiro.
There is another triangle variation which is much less common in BJJ. It is usually set up when the opponent is turtled. The turtle position is not as common in BJJ as in judo. One effective way to get your opponent to turtle is to threaten them with a guard pass. Usually if they have a very good guard which is difficult to pass, you can attempt a guard pass which will force him to defend by turtling up. When this happens, you have the perfect situation to set up a very dangerous reverse triangle.
This reverse triangle setup was seen at Bellator MMA hey a few years back when Toby Armada was able to put Jorge Masvidal to sleep with it.
BJJ black belt Gile Huni of Kimura BJJ Serbia shows a great set up that will surely catch your opponents off-guard: