Found this today. How to do a triangle setup from mount. Well worth a look.
For more information on how what a triangle is, see here:
A triangle choke, or sankaku-jime (三角絞) in judo, is a type of figure-four chokehold that encircles the opponent's neck and one arm with the legs in a configuration similar to the shape of a triangle. Applying pressure using both legs and the opponent's own shoulder, the technique is a type of lateral vascular restraint that constricts the blood flow from the carotid arteries to the brain, potentially resulting in loss of consciousness in seconds when applied correctly. Recent studies have shown that the triangle choke takes an average of 9.5 seconds to render an opponent unconscious from the moment it is properly applied.