When we hear triangle choke, most of us think of the triangle choke from the closed guard. See : Triangle Choke From The Guard
https://graciebarra.com/2017/02/choke-from-the-guard/

The triangle is a very versatile submission that can be setup from many different positions. As long as you can trap the opponent’s head and arm together with your legs, you have the basic ingredients for a triangle finish.

Let’s take a look at how some Gracie Barra instructors attack the triangle variations in some creative entries.

1) Inverted triangle against the turtle / Ataque do adversário de 4 Apoios – Triângulo Invertido
fans of Olympic judo will be familiar with the this entry when you are in head to head turtle position. Many international level judo matches have been won with this entry.
https://www.youtube.com/watch?v=FpAuxMN01QM

2) Armlock attack from the inverted triangle / triângulo invertido com variação para o arm lock
This is a variation of technique #1 with a more dynamic entry after sprawling to defend the double leg shot. Nice armlock finish by Professor Rodrigo Canale!
https://www.youtube.com/watch?v=ADTFj9G5oiY

3) Attack the turtle from the back triangle / Triângulo partindo da pegada das Costas
As an alternative to attacking a crucifix when the opponent attempts a single leg grip, you can enter into a triangle from the back.
https://www.youtube.com/watch?v=XKB3Gt20uhI

4) Variation #2 / Triângulo partindo das costas #2
I love this entry to the triangle. It is a variation of technique #2, but note the finishing position is very different. The triangle is finished from guard.
https://www.youtube.com/watch?v=5KXGs8f_n3Q

5) Transition to inverted triangle from armlock / Transição do Arm Lock para o Triângulo Invertido
Bet that you didn’t see this one coming and it will surprise your opponent too! Prof. Lucio shows a surprise attack from when your opponent is defending your armlock from the top.
https://www.youtube.com/watch?v=azcDjkLZR3A

See also on Gracie Barra : GB Techniques: Triangle Choke From The Guard
https://graciebarra.com/2017/02/choke-from-the-guard/

Credits: Mark Mullen
Gracie Barra Black belt based in Saigon, Vietnam
Instagram: facebook.com/markmullenbjj