You knew your opponent was going to play followers next turn, had a number of cards in deck equal to 8mod10 (i.e. 8, 18, 28, 38 etc..) and there was 1 estate & 1 province in supply, and no extra buys. If you gained the Duchy, they could gain the estate and province, increasing gardens value and winning. If you gain the estate, they cannot boost their Gardens that turn, and a Province loses them the game. Next turn, you buy the province and win.