I was looking for a subject and came across Bill Vigil performing one of my favorite wrist locks which I have used many times while taking someone into custody, it never let me down. Bill does it perfectly so I will borrow it rather the redo it. His explanation is sound, I will add a personal preference of how the pain is created. The cross wrist grab is neutralized when you place your hand over his hand clamping it to your wrist, your hand comes around like a snakes head over his wrist to his center line pointing toward his chest causing him to drop to his knee or on his butt in much pain. Here are a couple secrets to increase the pain, as the index finger points the real action takes place with the bottom fingers on the pointing hand. You create tension between those fingers and about 4 or 5 inches up that wrist which causes your outer bone to flex and dig into his wrist causing more pain. Sometimes I would hold the wrist close to my chest an could apply more pressure if I lean forward. Try it, work on it and figure it out, it is very fast and painful, many will cry like a baby. Let me know how you do.