This is a variation on making a screenshot selection.
- First, hold down Command (⌘) + Shift + 4, which will bring up the screenshot selection tool.
- Next, hold down Control, and make your selection on the screen using your mouse.
- Then, pull up whatever document you want to paste the selection into, and press Command (⌘) + V.
That’s it! If you frequently make screenshots and insert them into documents, this should streamline your workflow a bit.