Having the underhook on top is better because if they peel off the top hand, your choking hand is free to attack. Another reason is because it's a stronger grip because it's harder to peel off the top hand. It's also trickier because people usually expect the underhook to be on the bottom. One reason the seat belt works better than grabbing their shoulder and wrist is the fact that the choke lines up better. With the seat belt you choke them with the blade of your wrist, the other way you choke them with the inside of your elbow.