Today in class we were discussing a video (below) by Keenan Cornelius because it highlights an important concept--namely, the idea that when we are on bottom, we have multiple levels of defense. Several structures can be put in place to keep one's opponents away using different configurations of one's hands, wrists, arms, head, hips, knees, shins, feet, etc. If one lapse's for just a moment and one of these layers of defense fails, one's opponent can exploit this architectural failure to initiate the pass. The solution: If possible, place yet another layer in place on top of the lapsed layer and start battling for positional safety once again. This notion of always placing structures between oneself and one's opponents is crucial and foundational and yet I think that many practicioners, especially at the lower belts, don't pay enough attention to precisely what it is you're doing in this context and why. It's why their defenses are so easy to exploit and defeat during live training. They are structurally unsound. That said, here are some videos by Keenan Cornelius, Ryan Hall, and Saulo Ribiero that I find useful.