Here Rodrigo Gracie demonstrates an Armbar setup from the top position.
The technique starts when you have the top position and your opponent rolls onto their side in an attempt to escape. When your opponent is on their side it is very difficult to maintain the top position so you have to transition to a position that I have heard Rickson Gracie call the 'Technical Mount.'
When you arrive at the TM position your opponents arm is right there for the taking. Rodgrigo traps the arm and grabs his collar to secure his opponents arm. This is a great technique that can easily be modified for no-gi.
Basically all you have to do is catch your opponent's wrist with the inside of your elbow. Then continue with everything just as Rodrigo explains it. The next thing you know your opponents arm will be all stretched out ready for the tap.
