The remarkable power of underhooks in ground grappling: It’s never enough to just pin someone with body in Jiu jitsu. To exert meaningful you need to go further and anchor yourself to your in ways that
– shut down his ability to escape
– set submission
– facilitate moving to another pin
Probably the single best best thing you can do to achieve these things is to underhook your arm. This immediately anchors you to your opponent in ways that make escape difficult and easy. This should be one of the first things you look to attain once you get to half guard or any pin except north/south. Don’t settle for just pinning – get your underhook as soon as possible and you’ll double your effectiveness from top position