Quick answer: The player who draws a Jack says “Never have I ever…” followed by something they have never done. Players who have done it drink.
What Does Jack Mean in King's Cup?
Keep the prompt light and respectful. Nobody should have to reveal private or uncomfortable information, and every player may pass without explanation.
Common Jack House Rule
A common alternate Jack rule is Make a Rule: the drawer creates one simple rule that lasts until replaced.
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.