A card is placed and if two different sides touch different cards and their adjacent values add up to the same number,(e.g. 5+6 = 11 = 7+4) the opponent's cards used in the Plus process are flipped. At least one of your opponents cards must be involved in the Plus process, it is useless to try and Plus your own cards.
Below is an example:
Plus is also able to lead into Combo, see Game Rules section for the Combo rules.