Spring-loaded folders, which have been part of the Mac OS since before OS X, let you click and drag files or folders. When you let the mouse pointer hover over a folder, the folder will open to display its contents. You can quickly and easily drill down through folders to locate a specific folder or file, and then click and drag the file or folder to its target destination.
Because the 'Trash' icon in the Dock is really just an alias to the Trash folder on your hard drive, it responds to spring-loaded clicking and dragging. It will magically open a window that displays its current contents, just like any other folder would. You can take advantage of this capability to view the current contents of the Trash while you drag items to it.
Spring-Loaded Trash in Action
Make sure your trash already contains some debris.
Click and drag a file you want to toss to the 'Trash' icon in the Dock.
Let the mouse pointer hover over the 'Trash' icon for a second or two.
A Trash window will open on your desktop, displaying the current contents of the Trash.