The sound of spitting is always sweet to a Norwegian. ‘Tvi, tvi’ was used to put a curse on someone that you liked. The superstition was that if somebody had already cursed you then the evil spirits had no reason to bother you. It was usually used as a friendly good-bye, or a send off to a journey.
Nowadays the expression is used when wishing someone good luck before they go on stage. Instead of saying ‘break a leg’ in Norway you say ‘tvi, tvi’.