There are only a few positions where you can pin the opponent off your back, and the Diamond Pin is one of them. It comes off playing a strong over hook guard, where you threaten the triangle choke enough to make your opponent want to keep his hand close in front of him, allowing you to cross pin and setup the Diamond pin.
What’s great about this pin is that you can elbow the hell out of your opponent, with easy access to his face. In MMA, this can be a fight ender, as you can cut him up bad. When he starts trying to escape, he can put himself into a triangle choke, a bent arm lock, or a regular arm bar – so there is no easy way out.