The Marce is basically a Darce that you hit from the B-Fly Guard. In the video Sean Bollinger goes through the tech step by step.
He shows an armdrag setup to the Marce and explains that the main thing is to get your opponent's arm across your body which makes it easier to get the angle that you need to setup the choke.
The first control position with one hand gripping your opponent's neck while the other one grabs the wrist of the hand at your opponent's neck is an important detail that gives you control through the whole tech. You could go to the arm triangle position as he mentions. Even though this may seem like it is a stronger grip it actually give you less control.
