The Body Triangle in Jiu-Jitsu is one of the absolute worst positions you could end up in. Not only is your back taken, but you’re experiencing this tremendous pressure across your ribcage… That makes you want to puke and tap out immediately. Which is why having a solid Body Triangle defense is so important.

Here’s a simple escape from this position, when you fall on the overhook side (of the Seatbelt grip) and when the Body Triangle is locked on top.
Gordon Ryan demonstrates and explains.

SIMPLE BODY TRIANGLE ESCAPE

First things first: you’ve got to deal with the training partner’s choking arm. This means that you’re going to establish a thumb-inside grip on their overhook wrist.
If, at any point throughout the remainder of this technique, your partner uses their second hand to peel off your grip, stop everything. And use your other arm to help and reestablish your first hand’s defensive grip. Only then should you continue on.

Now that your primary defense is in place, it’s time to deal with the actual Body Triangle. You could try scissoring your leg over the training partner’s top foot, but if its glued to your buttocks, you won’t succeed.
In this case, use your secondary arm and position it under the partner’s bottom knee. Lift their leg up and sprawl.

From here, scissor your legs and you’ll find that it’s incredibly easy to position your leg over the training partner’s foot. Then, square up and go forward with the remainder of your back escape.
Gordon Ryan demonstrates the finer details of this technique on the video below: