The [CSS] Ninja has written up a great article describing the process of not only how to do it (in javascript), but also show us how to actually track down the information. Tracking down the information on a certain feature especially in Google's case is very important since they obfuscate all of their javascript.
We also get a Live Demo of the Drag and Drop feature along with the source files.
The [CSS] Ninja is a great resource to have in your toolbox, make sure to subscribe to the RSS feed