Quick answer: The player who draws an 8 chooses a mate. Whenever one partner drinks, the other drinks too.
What Does 8 Mean in King's Cup?
Eight means “mate.” The partnership normally remains active until the next 8 is drawn. Decide whether actions caused by a mate can trigger each other repeatedly; most groups say no.
Common 8 House Rule
For non-drinking play, mates share points or complete challenges together.
House rules differ by group. Confirm the meaning before the first card is drawn, allow anyone to choose a non-alcoholic alternative, and never pressure a player to participate.