The proxy icon, the small icon in a program’s title bar, can perform a few handy tricks. There are many applications that use proxy icons, both Apple and third-party, such as TextEdit, Preview and Photoshop.
Normally clicking and dragging from the top of a window moves the window around on the screen, but clicking, holding on the icon for a second, then dragging picks up just the icon.
Most applications behave similarly when you drag the icon, and also when you add the Option modifier key. Dragging without a modifier key and dropping the icon either into a Finder window or to the Desktop creates an alias (indicated by the cursor turing into a ‘curly’ arrow) to the file which is open in the window you dragged from. You can also drag and drop onto an application’s icon in the Dock to open the file with that application. One last thing you can do is drag into an Open dialog or into an email or chat window to open the file there.
Holding down the Option (?) key while you drag creates a copy of the file rather than an alias to it, indicated by the cursor changing to a green plus sign.
Note that this only works when you have a file open that doesn’t have unsaved changes. If you haven’t saved, the icon will be greyed out and you won’t be able to drag it. Also bear in mind that there are some third-party applications which do not behave in the same way. For example, in BBEdit, the functions are reversed, meaning that a simple drag and drop results in a copy, and to create an alias requires the Option key.