A triangle choke is a useful choke to use from the guard. It is a defense against someone trying to pass the guard (a).
As they try to pass, they put one leg on their shoulder (b). (In this picture, your right leg.) Unlock your legs and pull their right arm down and across your body (b) \& (c). At the same time you want to get the crook of your left knee over your right ankle (c). This is often hard to do. It helps to get your right shin horizontal and to slide your hips to your right. You may have to use your hands to lock your left leg over your right (b) \& (c).
Once you get the left leg folded over (d) \& (e) this is an incredibly powerful choke. Fold your left leg down while pulling their right arm towards your right shoulder (c) \& (e). Stretch your hips back (e) while holding their arm.
Extra pressure can be applied by releasing their right arm and grabbing the back of their head (d) \& (f). Pull the head down into the V formed by your thighs while stretching the hips out (e).
This is very much like a Kata Gatame, put on with the legs.
Their defense involves early on reaching with their left hand acroos your neck and securing the lapel (g). From here they drive forward and down to put a choke on with their forearm. It becomes a question of who does it best -- soonest.