Originally posted by: jack Spencer Jr
Caltrop? Are those those little things you throw on the ground hoping the guy chasing you steps on it and pierces his foot or something?

Basically yes, but they're not always small. Larger ones could be used against horses or automobiles. The conventional European style was also refered to as Crow's Feet, because of its shape.
The Japanese version that I've seen is a little different:

http://www.entertheninja.com/images/wpn_calthrops.gif

However, after your description of its placement in the film, that couldn't be its function.

I went back to look at the movie, and I think I've seen that everytime and said "What was that?" but not given it much more thought.

I'm with Dayv, it looks like some kind of arm extension, but it also looks like it may also work as a brace. That was a pretty hard impact and you'd think it would buckle any man's wrist or elbow otherwise.