I've always wondered about this stuff.  The wire contracts when you put a current through it.  Ultimately, that heats the wire up and it returns to the form that it "remembers" at that temperature.  very cool, I wonder how strong and fast it is to react.

Here's some links for those that might want to read more about this bizarro metal alloy.

http://en.wikipedia.org/wiki/Nickel_titanium

http://www.instructables.com/id/How-to-Form-Memory-Wire-Nitinol/

http://www.amazon.com/s/ref=nb_sb_noss?field-keywords=muscle%20wire