Let me just say that their are a million ways to do this and we practice as many as we can, but this is about the easiest one I could describe.
First you take the striking hand on the outside with your same side hand. This could be from a block or just a straight up grab if you are fast and accurate.
We traditionally block farther up the arm and then slide down to the hand, but you get the idea.
Next is the hand placement. The fingers go around the ball of the thumb into the palm and your thumb on the back of the hand.
I usually do the next part with two hands, but this is really designed to be a single hand take. You are supposed to role the person's fist/hand up toward their body.
Note: You've got do this starting very relaxed or the person will tense up and if you've little hands like me then you are doomed. Away to assist into moving into this position is to dig your thumb between the two tendons - ouch!
This is first part of the pain. A person can be dropped to their knees in this position if the curling towards them is continued while close in. Not very descriptive - sorry.
And finally the twist. It's not a great shot of what's going on, but let's say that not only am I pushing his knuckles towards him, but I'm rotating his hand so that the ball of his palm is now going towards his body. You'll notice that his elbow is moving in towards his body to create relief. The only escape is take a fall or other counters that take to long to describe.
I usually have to do this with two hands. It reduces slippage.
And, in other news, Nilon is absolutely obsessed about testing for his next rank. He constantly asks Teacher about it during breaks and after class. I find this repulsive because it goes against my whole gut feeling about why we are there, but at the same time I'm absolutely drawn in because I want to know too! So Teacher says, "we are absolutely testing before the end of the summer." He pointedly looked at me...