What is the encounter you're referring to?
Lurker's Other Worlds Card, yellow, Yuggoth on top: swap current Stamina and Sanity totals.
It seems this is just a way of calculate the amount of eventual losses to one one of the characteristics, but interpretation is kind of open.
The original post should be in the Lurker's forum, but I did not notice before the card had the expansion logo: sincerely sorry and I do not have the permission to change foruns, only a moderator.
I keep playing single investigator games, but find that adding a few Lurker's components is worth it for improved variety, especially the Gate cards. But I'm still waiting for the deck with Arkham encounters that was missing!