Opponent's stones can only be pushed with the Aggressive move. An Aggressive move cannot push two stones at once, and cannot push a stone of your own color.
EXAMPLE: White makes a 2-space diagonal Passive move (a). White's desired Aggressive move (b1) cannot be made, as two stones block the move. With this Passive move White's only Aggressive moves can be (b2) or (b3).
IMPORTANT: If an Aggressive move cannot be made, the initial Passive move is also invalid. In this case the player must select a different Passive move.