
One of their legs is trapped between yours. Get on your side, win the underhook, and don't get flattened.
Half guard is where many guard passes stall, and it is a strong place to fight from if you get it right. With one of their legs trapped you can stop the pass, recover full guard, sweep them or come up to their back.
Everything depends on not getting flattened. Turn onto your side, win the underhook and keep your head off the mat. Get those two things and the sweeps (the old school, the knee lever, the deep half entry) start to work. Lose them and you are fighting to survive.