The fundamental idea of leading through strength is to help your
partner to make his high cards by allowing him to play after the
exposed hand. “Leading through” means to make a person play second
hand to the trick. You always lead through your left hand adversary.
Should your partner hold K J x with A Q x in dummy, it is obvious
that if you lead through the suit, your partner may win two tricks,
whereas if he is forced to lead up to the strength, one trick at
least would be lost. Players will often say, “How can I know that
my partner has any high cards in that suit?” Perhaps you cannot be
certain of the distribution of the high cards; in leading through
strength you hope that your partner holds the missing honours,
realising that if all the strength is with the dealer, nothing
is lost. “Leading through strength” is simply looking ahead and
foreseeing what may happen. In other words, not placing your partner
in a position whereby he may be forced to lead up to the strength in
dummy.