The atlatl is a wooden handle about 24 inches long.  At the tip end is a hook, point, or pin.  It is used to cast or throw darts with great accuracy and tremendous force.   The darts are about 5 or 6 feet long and are flexible and look like oversized arrows.  The back end of the dart is hollowed out a bit so that it will fit over the pin on the atlatl.   This helps hold it in place but the dart is also held onto the atlatl with the thumb and first finger of the hand that is holding it in preparation for the cast.  (There are a few different ways of holding the dart in place.  This is only one example.)  The arm goes back and then forward.  When the atlatl reaches somewhere around the halfway point of the cast the dart springs off of the pin and flys into the air.

The atlatl has been in use for at least 20,000 years and predates the bow and arrow.  Compared to the atlatl the bow and arrow is a very new development.  The atlatl was used all over the world.  Some say that this very effective weapon was a major contributing factor in the extinction of the mega-fauna in the Americas which consisted of the large game animals like the wholly mammoth.
 
 

Below is an animated GIF that I made which will help you get an idea of how the system works. The process has been simplified a bit here.  In reality the atlatl would swing up and forward as it made it's arc which would add to the force achieved. But you can still see how an atlatl moves a dart.  I can almost feel the push of that long dart as it springs from the atlatl.  I think I'll go get my atlatl and throw some darts right now!