Tactics: Double-Check

Double-check is the result of a discovery where both the moved piece and the disovered piece are checking the king.  Double-check is not always more beneficial than just checking your opponent, but it has the advantage that the king MUST be moved.  There is no way, in one move, to block both checks, or take both pieces, or take the one piece and block the other, so the only response left (if it's not a checkmate) is to move the king.  Beware that, depending on the position, it may be posible to get out of double-check by taking one of the checking pieces with the king, thus ending check from that piece, and moving the king out of the line of fire of the other.

Black is in check from 2 directions at once, even if he had more pieces, he couldn't stop both of them without moving the king.

Black can take the bishop, ending both threats in one move.  Just because it has a nifty name like double-check doesn't mean it's always a good move...