The remarkable power of underhooks in ground grappling: It's never enough to just pin someone with body weight in Jiu jitsu. To exert meaningful control you need to go further and anchor yourself to your opponents body in ways that
– shut down his ability to escape
– set up submission holds
– facilitate moving to another pin
Probably the single best best thing you can do to achieve these things is to underhook your opponents arm. This immediately anchors you to your opponent in ways that make escape difficult and control easy. This should be one of the first things you look to attain once you get to half guard top 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
