Video is self-explanatory. Figured a lot of you guys would want to see this.http://www.smoothben.com/video/video/show?id=3192188%3AVideo%3A5227
Skip to about 1 minute in to see the technique on display.
This is why it's very important for the person putting the guy in the Omoplata to have control of more than just the arm itself. You've got to control the hips otherwise the omoplata isn't that effective because there is little to no posture control outside of the arm trapped. It's why you rarely (if ever) see a guy submitted by an omoplata no matter the position. A kimura from guard is a bit more effective although much more difficult to get started.
There is also another good omoplata escape which involves dropping down and rolling your opponent rather than posturing up and limp arming out