Download Complete Dialog
When a download is finished, IDM shows "Download Complete" dialog. You can choose from several ways to open the downloaded file.
You can turn off showing the dialog by checking "Don't show this dialog again" box. Later you may turn it on or off in "Options→Downloads" tab
At bottom-right corner of this dialog has "drag & drop" icon to start drag & drop operation for a downloaded file. Sometimes it's very convenient not to open the file directly, but drag it from IDM and drop it to another program.
For example if you are using a music player program like Winamp to hear MP3 music and if you open a file with "Open" button, the player will clear the current playlist and open the file alone. But if you drag & drop the file to the player, it will be added to the current playlist.