In general the 10th Planet style of finishing the arm triangle/side choke tends to be a little more cranky and arm squeezy than what I prefer, but it obviously works great still. Almost everything in jiu-jitsu is personal preference and that's why it's so cool. If their left arm is trapped, no matter what grip you're using, try to focus on getting his left shoulder to roll into the side of his neck. That's going to close off the artery, so I would say to twist his trapped shoulder more to your left / his right, this way it will drop into the side of his neck and put him to sleep. The trapped shoulder is always the problem side.
The open side that you're closing off with your own arm is almost never an issue. This goes for any leg or arm triangle (triangle, reverse triangle, inverted triangle, side choke, darce, anaconda....)
Watch the first 90 seconds of this video. This might be what Eddie meant by twisting. if not then hopefully you can at least pick up something from the video. This preview clip also has a killer arm-in RNC/nogi ezekiel finish at about 3:50. It may be in a gi, but the entire dvd is nogi friendly, and Hall is obviously an incredible nogi athlete, and this dvd helped out my side chokes, darces, and other head and arm chokes so much. Definitely worth the money.