"We have two weapons", intoned the wizard. The evening sun dappled the stone chamber. "Each of these weapons has magic force. Sometimes they have power on their own and sometimes their forces combine."
"What are these weapons, Dumbledore?" quizzed Harry.
"They are STRENGTH and LENGTH, young Harry. Observe." The wizard waved a wand and 13 cards tumbled before Harry's face, jiggling and giggling. "Get a grip," moaned the wizard. The cards obediently fanned themselves in the air. This is what Harry saw:
"This hand is powerful as a result of its STRENGTH in high cards. But note that it has no shape. Opposite the wrong hand it may very well take no more than 4 tricks despite its power. But that would be extremely unlikely in practice. We open 1 Diamond expecting to rebid 3NT if partner responds. Now consider this hand:
"This hand is powerful as a result of its LENGTH in the Spade suit. It has no power of STRENGTH. Now we must use its power quickly and make a pre-emptive strike before the opponents muster their forces: we must try to cut their communications by opening 3 Spades!"
"Wow, that seems a lot," said Harry.
"True. But consider: if we bid like that before they bid, we hope fatally to damage their auction. We take away precious bidding space. Can they bid the right game? Will they miss slam?"
"But what if we have a hand with both powers combined?"
"A very wise question, young Harry," beamed the wizard, snapping his fingers to change the cards. Harry saw:
"Here, we have Strength and Length. There are 16 points and a 6 card suit. But the value of the hand is worth no more than a 1 Heart opening. If, however, partner responds 2 Hearts we now re-value the hand for it's SHAPE. The singleton pulls its weight so we ADD 3 points giving us a total of 19: enough to attempt game. We bid a direct 4 Hearts! The STRENGTH & LENGTH combine but only after we find a fit!!!"
It really isn't magic at all, y' know...