In this "Emacs Rocks" video relating to the library
multiple-cursors ( https://github.com/magnars/multiple-cursors.el ), the author demonstrates a way of swiftly performing calculations, getting the result in the main window (not in the mini buffer). However, he does not explain what commands are being used. I haven't been able to replicate them, having consulted the available documentation.
Any ideas on how he does it?