Cop's Armpit is for grabbing loose, grabable clothing. This is especially the case if the Badguy's clothing is so baggy that getting your hand inside the armpit to the bicep from Calm Down Sir to Cop's Russian could be a problem.
From 3:00 or 6:00, find the the seam along the armpit of the Baduy's jacket and grab there. As the Badguy moves his captured arm to punch, you can catch his wrist with Swan's Neck or simply grab the cuff of his sleeve.
The armpit seam is such a good handle that you don't even need to immoblize the elbow joint with your core. This allows you to fight from a greater distance and use kicks if you like.
You can enter into Cop’s Armpit using Calm Down Sir or you can transition from Cop’s Russian and other 2-on-1’s.