Sweep Option from Triangle stack. Keep in mind this is an OPTION which works rather well IF your opponent keeps trying to stack his way out of a triangle. Note, that he must be intent on stacking for this sweep to work, if he drops back down to react to the sweep just reposition and sink the triangle in deeper. (The "what if" options are always there, but you'll notice that its rather effective if you have the flexability and speed to pull it off)
I'm attempting to set up a triangle on Seth. In an attempt to get out of my triangle, Seth will try to stack me.
I unlock my ankles and transition to a modified rubber guard by holding my shin with my left hand.
Simultaniously I'm working to press off his hip and create space to work my ankle in behind his knee. (space created to show position)
What the space should look like. Keep your knees tight and your calf pulled over his neck to prevent him from escaping.
Using the angle I created to get my ankle under his knee, I replace my left arm with my right arm and continue to keep everything tight to limit any chance of escape.
To hit the sweep, I lift my ankle trapped under his knee and pull his foot toward my body. Pulling his foot will make for easier leverage than pulling his ankle.
Sweep in progress