I think I have probably gotten the reverse arm triangle the most out of all the submissions I have ever done. I seem to hit it in two main spots...twister pass when in half guard (im on top) and side control while transferring to north south then back again to side. chris gives a perfect explanation in his video above. for me its all about using my chest to manuever the arm where i want it and keeping keeping his arm pinned to his chest.
The one tip i can give that has worked for me...if for some reason I cant finish it and my opponent is flat on his back...i release the hand off my oppnonents head and grab the back of my own head and lift my head up slightly. This engages my chest and puts all my weight and pressure on the trapped arm. Just something i found that seems to work for me.
Getting your opponent flat and the arm\shoulder trapped in the right spot (right next to the corotid) are the key...once you have both of these you should have it.