(I figured out you don't have to pass anything specific into the share parameter, you can do "?share=poop" or even just "?share."
Also, this parameter used to (maybe still does) set a cookie so that you can browse around the rest of your session without worrying about a sign up modal.
(I figured out you don't have to pass anything specific into the share parameter, you can do "?share=poop" or even just "?share."