Before playing your second-last card, raise the flag to let other players know you are going to win in the next round.
When playing with physical cards, you would say out loud "Last" while throwing your second-last card, or "Mau Mau" if your last is a Jack (more info below). This gives other players a chance to do something to stop you. Note that you have to raise the flag every time you are in the winning position, not just the first time.
When the game ends with a Jack, we call it a "Mau Mau" victory and all results are doubled. A special Mau Mau flag is displayed to other players if you announced a Mau Mau victory, instead of the ordinary flag. They can try to stop you from winning if they play another Jack at you (because Jacks cannot be played on Jacks) or a penalty card.
Expert info: The game can also be ended with multiple Jacks, and the results are tripled, quadrupled etc. Here is how you can do it:
If you get your turn with only a few Jacks left and another non-Jack card which matches the table, raise the flag before playing your remaining non-Jack card. Now wait for your next turn.
If you are lucky and you get your regular next turn, play your Jacks one after the other (the game will allow it) until you reach Victory.
If you forget to raise the flag and you play your last card thinking that you won, the Croupier will send you two penalty cards and the game will continue normally.
Expert info: When the last winning card is played, if it is one of the penalty cards (e.g. a Seven), penalty cards are served to the next player before results are counted.})}