There are 2 cards of a month on the ground. Now you put down one Hand card to match one of them and you flip the top card from the Flip pile that happens (or your kind of strategically guessed?) to match the other card of that month, meaning now there are 4 cards of a month on the ground by your play. You not only collect those 4 cards but also receive one bonus Plain card each from the other players' Collection square.