Depends on how they pass.
If they are going for the kimura and they have their back facing your head, but you also have the underhook, their balance is going to be weak behind them. Imagine a line going perpendicular to the blue guy's shoulders; this is his weakest direction in this position. You can give up the half guard and either get a butterfly in or just explode and push them backwards since they have a weak base in that direction. The timing has to be right and you have to commit, but other than that it's hard to screw up. I've landed reversal as a blue belt against brown and black belts.
If they are passing facing you, you can do the normal scarf-hold reversal. Again, you have to do it quickly before they establish a side control base.