Here's what's posted on the What HiFi website, they explain it much better then me.
"The simplest hook-up between an amplifier and a pair of speakers uses a two-core cable per speaker - one conductor is designated positive, and marked with a '+' symbol and/or red plugs and terminals. The signal goes into a crossover in the speaker: an electrical circuit that splits it between the drive units. So in a two-way speaker, the bass and midrange signals go to the woofer, while signals above a certain frequency (often around 3kHz) go to the smaller tweeter. In a three-way speaker, the signal is split into bass, midband and treble. Bear in mind, however that a speaker with three drivers may not be a three-way speaker - the term purely refers to the way the signal's split. Mission's 753 Freedoms, for example, use five drivers, but are in fact only two-way speakers.
But we digress - back to the crossover. One of the problems with a standard crossover is that since one pair of cables carries the signal to and from the amp, bass signals can tend to swamp the more delicate treble stuff, especially in the return (or negative) connection back to the amp. By separating the signal all the way back to amplifier's output stage, using two runs of cable or a purpose-made four conductor biwire cable, these effects can be greatly reduced. To do this, you need speakers designed for biwiring. That means the crossover needs to be completely split, without the common negative found in standard design.
To all intents, a biwirable crossover is two separate circuits - one to filter the treble out of the signal for the bass driver, the other to prevent the bass from reaching the treble driver. The dead giveaway is four terminals - two positive, two negative - on the back of a biwirable speaker. No multiple terminals, no biwiring - sorry, Stuart, that means it's not worth ripping your Aiwa speakers open!
To allow them to be used with a single two-core cable, the terminals on biwirable speakers are usually connected by some kind of jumper bar or link - the speaker instructions will tell you how to remove these to prepare the speaker for biwiring. That done, you need two positive and two negative connections between the speaker and the amp. Whether you use two separate two-core cables or a four-core biwire run is a matter of taste, but most users will find the latter tidier. At the speaker end, the four conductors - two positive, two negative - are connected to the terminals, but what happens at the other end depends on the outlets the amplifier provides.
With the possible exception of amps with springclip terminals, which have trouble accommodating even single cables, any amp can be used for biwiring. Some provide twin sets of speaker terminals specifically designed for biwiring, while others have A and B speaker outlets you can use to the same effect. If the latter's the case, you need both sets of terminals turned on all the time.
An amp with a single set of terminals (ie just left +/- and right +/-) can also be used: the two positive conductors from the left speaker are connected into the left '+/red' terminal, and the two negatives into the '-/black'. Repeat this for the right speaker, and that's it. If you have an amp with 4mm banana socket terminals, most cable manufacturers and dealers will supply biwire cables 'commoned' into a single set of plugs to make this even easier. If, however, you have to use bare wire connections or are soldering up your own cables, check and double check the polarity of each amp-to-speaker connection - it's all to easy to get, say, the left tweeter out of phase with both the left bass and the right speaker. You won't do any damage to either amp or speakers, but it'll sound pretty odd. Get it all right, and what you should hear is better focus and clarity in the treble and (particularly), plus better soundstaging. But you can go further...
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You can of course bi-amp, but I don't want to go there, that's a whole other topic. and there's also bridging the amps...........
And Yes, I bi-wired my B&Ws
