It looks like adding the Ctrl copies it to the clipboard after you select it - without the Ctrl it'll save it to your screenshots folder (I think the desktop is default, but I have mine set up to go into a Dropbox folder).
Other screenshot shortcuts:
Shift + Cmd + 3 - screenshot full screen
Shift + Cmd + 5 - complicated screenshot (I use this for quick videos)
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It looks like adding the Ctrl copies it to the clipboard after you select it - without the Ctrl it'll save it to your screenshots folder (I think the desktop is default, but I have mine set up to go into a Dropbox folder).
Other screenshot shortcuts:
Shift + Cmd + 3 - screenshot full screen
Shift + Cmd + 5 - complicated screenshot (I use this for quick videos)