First of all I don't know what to call this move, thats part of my question. But also I'm not sure if its even a lock. Right the attacker grabs your wrist, you place your free hand on top of the attackers hand (the one of your wrist) then hook the subdued hand around the attackers arm and push down and towards yourself. This creates pain in the attackers arm and makes them, in extreme cases, lower onto their knees. I like this move and I am curious to know the name of this move and whoch martial art it comes from, also if there are any other small moves like this

This one one of the early things my teacher taught me, it's a great party trick

My teacher is mostly a go-ju ryu practicioner so as far as I can tell the move is of okinawan decent, though looking at it from a chinese point of view I'd imagine that it's a fairly early developed move that perhaps came from the shaolin schools.

It sounds like an Aikido/Jujutsu technique, nikyo:

http://www.aikidoonline.com/Features/images/Sheahon/Sheahon.nikyo.jpg

It can be very painful if done correctly but you really need to catch your opponent unawares for it to work effectively.

No that's not it, the end result resembles more a person falling over on to the front amd getting their arm trapped extended out to the side (roughly).

What he described is a nikkyo from a cross wrist grab as opposed to a collar grab shown in the pic. Nikkyo can easily transition into a straight arm prone shoulder pin.

It sounds to me that he is describing a center lock (I know not traditional name). Sorry, googled but found no pictures. It is found in hapkido which derives from daito-ryu so I suppose its origins are in Japan.
Center locks are wrist centered locks as opposed to arm bars.

isnt that whats known as a "key lock"?

There are probably many non-traditional names for it.
Umm, is this anything like the hold you are describing?
http://images.google.com/imgres?imgurl=h...ficial%26sa%3DG
I looked, this is the best picture I could find. Their positioning is a bit different then normally with center locks so it may seem a bit different.

I think i know the lock your describing it can be continued into a throw. I believe it to be predominately an Aikido move.

It's shown in Under Siege 2 when Rybacks niece demonstrates it on one of train's staff ( a porter I think) early in the movie before the bad guys take the train.
It can be seen at 2mins 30secs on this clip.

http://www.youtube.com/watch?v=Y_kjVJsnlJo

I know at least she's pretty to look at for 2 and a half minutes.

Yes, that is a center lock, but it is going at a more sideways angle as opposed to straight down. I do not know about aikido but the throw is found in hapkido and later in the movie. I would assume it not to be in aikido since aikido focuses more on full body throws.