Mook Yan Jong (木人樁 lit. wood man pile) is a wooden dummy used in practicing martial arts. Most frequently associated with Wing Chun.
While originally made out of wood, wooden dummies are now also made out of steel and plastic. No matter what material is used, they are all still called wooden dummies.
The wooden dummy has three arms and one leg. This is sufficient to represent a human body in various positions, for the purposes of training.
The dummy can be mounted several ways. Mounting the dummy on two slats of wood and suspending it provides several features. For example, the give provided by the wooden slats provides a certain feel that is similar to a human's involuntary reaction. The bounce back allows practicing absorbing energy into the stance.
The whole body is used in practicing the dummy. All of the principles evident in the three Wing Chun forms 'should' also be evident when practicing the dummy.
Jackie Chan can be seen training on a steel dummy in Gorgeous http://www.imdb.com/title/tt0184526/ (near the end of the movie) and on a wooden dummy in Rumble in the Bronx http://www.imdb.com/title/tt0113326/ (near the start of the movie).