The Onion Skin tool in Flash

Basic frame by frame animation of any complicated movement is, not surprisingly, daunting for first time animators. Adobe Flash gives users a huge number of tools for making this process easier. Often times these tools are analogous to what traditional hand drawing animators use. Traditional animators use light desks and translucent paper, allowing them to see through their current drawing to see the previous ones. 

Traditional frame by frame animation with a light table and translucent paper.

This makes drawing a complex animation much easier because you can immediately grasp how the new frame should look in relationship to the previous ones. But how can we do this in Adobe Flash?