Quick answer: Everyone starts drinking together. The player who drew the Ace stops first; then players may stop one at a time in clockwise order.
What Does Ace Mean in King's Cup?
The Ace starts a Waterfall. Once everyone begins, each player waits for the person before them to stop. Nobody needs to finish a drink, and small sips or a non-drinking action work just as well.
Common Ace House Rule
Use a pose waterfall: everyone holds a pose and may relax only after the previous player does.
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.