The word "potlatch" refers to an old tribal custom which involved the competitive exchange of gifts between two people, the idea being to 'outdo' the other person and thus enhance one's own status within the tribe.
M.K. Shibek has invented a variant of Time Travelers' Potlatch called "imaginary potlatch."
See examples of the game: http://www.surrealism-usa.org/pages/time.html.