BJJ is a grappling sport. You don’t punch or kick anyone, but try to take them down from standing, and put yourself into a dominant position over them. They attempt to escape that position. If you really get them, they tap to admit defeat: you’ve submitted them. That is, from another perspective, you’re taken down. You’re put in a painful position. You attempt to escape. When you start, your job is to suffer. You get crushed and choked. Advanced players put you into positions where they could easily break your arm. You’re killed over and over again. And it’s great.