Before we show this one, a disclaimer: You can really hurt someone with this, fast so train it responsibly as hell. If you’re a new spazzy white belt, or seem to accidentally hurt people with regularity, watch it as grappling **** only at this point.
Some submissions are about feeling it out, and the butterfly armbar is certainly one that is about sensing the timing more than finagling position for 40 seconds until it clicks into place like a stubborn Lego. Those who master it are genuinely practitioners with high “fight IQ,” aka folks who can shut their eyes and feel a submission opportunity based on understanding of body mechanics.
Your window for executing this comes when an opponent reaches for the underhook. While blocking their far shoulder, move your foot onto their hip and grab the advancing elbow with your palm firmly. Once you have the elbow and can post against their hip, lean back and and close your knees:
Feel free to green light anyone using this one in the gym to be a d***, PS. Elbow injuries take too long to heal to tolerate people using this carelessly.