To fart into the open mouth of someone who has passed out, typically from consuming excessive amounts of alcohol. The lafrance is a party challenge: if a party patron has been so stupid as to consume more alcohol than their limit and to pass out as a result, other party patrons are welcome to try to position their posterior over the mouth of the passed out person and fart.
The origination of the lafrance may have been influenced by the movie Monty Python and the Holy Grail where a French soldier tells the English that he is going to "fart in (their) general direction."
Bill passed out with his mouth open. I'm going to go lafrance him!