The secondary-selection is used when the cursor is in some focussed Recipient window, in order to grab some text (or whatever) from some Donor window (possibly the same window as the Recipient) and have that text pasted at the Recipient’s insertion-point (overwriting any primary-selection in the Recipient).
Never even thought of that, but this is genious.
Maybe using ctrl for that is not the best decision, but I understand this was just a showcase. I wish this actually existed and was used in real desktop environments.
It’s in emacs! Using alt instead of ctrl https://www.gnu.org/software/emacs/manual/html_node/emacs/Secondary-Selection.html