Bi-wiring is running 2 sets of speaker cables from the amplifier to the speakers. Two sets of binding posts are necessary on the speakers to accomplish this. One is for the high frequencies. The other is for the lows.
The audible advantage is better imaging. The difference is not huge, yet it is usually the cheapest upgrade to any system... by far.
A better yet more expensive solution is bi-amplification.
a good technical explanation
BI-WIRING AND BI-AMPING MISCONCEPTIONS.
NEW EDIT BY Yuri Halberstadt of YURI~BRAND NEW AUDIO (www.yuribrandnewaudio.com):
BI-WIRING is not a real and audible advantage (read on to find out why). Worse, it is sometimes pure horse crap, namely when both wires are re-joined inside the speaker cabinet before they enter the crossover network. Double binding posts on speakers so wired are a hoax.
When done properly though, the crossover network inside the cabinet is separated in a High Pass section and a Low Pass section that don't connect in any way. However, even then the path of the signal is not significantly altered as compared to entering a crossover network PCB and being split only then into it's HF and LF. The amplifier does not 'see' a difference so to speak, between splitting HF and LF at the amp output or at the speaker input. Total impedance is (virtually) the same, function of the XO remains quite identical.
The only advantage would be the aforementioned influence of the magnetic field around the speaker cable, as generated by the current of the integral signal, on the HF component of said signal while flowing through said cable. Now search the internet. You will find no academic literature mentioning any evidence that this has an audible effect anywhere. Not for lack of research mind you, but for lack of effect. What you will find, are explanations as to why the behaviour of the crossover system and the amplifier remains unaltered by feeding a signal through a single cable as opposed to a double cable. The resistance may be a bit lower, but if you're serious about your sound, you already had sufficiently low resistance cables to begin with. In short, most electronics engineers will laugh at the concept, because it is a non issue. Don't mention it when you're among audio professionals: you'll be identified as ignorant.
BI-AMPLIFYING is a completely different matter. Here the HF driver and the LF driver each have their own amp, offering some real advantages. The most important one in a passively filtered system (XO after the amplifier) would be HF performance is not modulated by LF transients (on account of voltage and/or current fluctuations in a shared power amp).
The real advantages though, are reaped when you switch to an active system, i.e. filter your frequencies first and feed them to the amps after (yes, you need multiple amps: x per x-way speaker). This can be done in a stand alone preamp or multichannel A/V receiver with DSP capacity, or with a stand alone components throughout. Preamp, DSP, multichannel DAC, power amps. This is why active multi-way speakers are on the rise in high end audio.
Your most important advantage, and you can bet your sweet ass this one is audible, being back EMF (electromagnetic force generated by the movement of the voicecoil, flowing back to the amp output) for each driver is compensated by the amplifiers return current in reaction to this back EMF, which conveniently is of opposite polarity. This effectively cancels the back-EMF, resulting in total elimination of an important form of distortion: back EMF modulating the signal and the cone movement.
In other words, back EMF is a form of feedback that generates the signal needed to eliminate itself: movements of the cone caused by its own mass inertia generate a current that comes back in opposite polarity, counteracting exactly this movement.
The same goes for sound reflected inside a cabinet (i'm against cabinets, but anyway): especially at mid and low frequecies the approx. 50% of the energy radiating off the back of the cone will be reflected inside the cabinet and (a large part of) its pressure will modulate the cone movement. This cone movement, not being present in the signal, will generate back EMF that, yet again, immediately compensates itself.
All the time assuming, of course, the output impedance of the amplifier is low enough to let the back EMF flow freely through it, so it will come back at more or less the same magnitude, effectively cancelling itself almost completely on account of the polarity switch.
This explains both why output impedance is an important parameter for assessing amplifier quality (the lower the better is the logical conclusion, extremely low is best, Hypex rules) and why passive filtering is not entirely adequate when SQ is of the essence.
The effect is most significant in the LF, where pressure radiated inside the cabinet is hardest to damp or absorb and where mass of the driver is highest, resulting in more unwanted movement due to inertia.
Read more on no nonsense ultra end audio at www.yuribrandnewaudio.com.