So I've been catching some triangles in different positions lately, and have been stuck on the proper ways to finish or if even possible. One is if I'm on bottom in north/south. I can catch this sometimes and try to sweep with it, but I don't really understand how or if I could catch a submission with it. Also, if someone shoots on me, I can catch them as they drive into me. So it's almost like I'm sitting on their head looking towards their heels. I can finish this by grabbing their ankles and arching, but my buddy/training partner told me that it was hurting his back which is why he tapped, so I'm not sure if that's even legal. What's the proper way to finish that one? Thanks!