This explanation is from TAS The Golden Ear's Guide to Audio system Setup and Evaluation. The except in the book is quoted from Bill Low of Audioquest.
"When you're using a single full-range speaker cable (not bi-wiring), the large amounts of bass energy carried within the single cable has an adverse effect on the upper frequencies. In a bi-wire setup the cable feeding the the higher frequncies no longer handles the large magnetic fields caused by the high current needed to produce bass. Bi-wiring does not affect the bass fundamentals, but the treble signal now travels a less disturbed path. It's a little like the differnce between swimming through waves versus through smooth water. Bass frequencies will sound better because bass definition is actually located in the midrange and higher frequencies. It is always wothwhile to take advantage of the benefits of bi-wiring when the speaker manufacturer has gone to the extra expense of providing this capability. The performance benefits of reducing distortion in this way are substantial."
"When you're using a single full-range speaker cable (not bi-wiring), the large amounts of bass energy carried within the single cable has an adverse effect on the upper frequencies. In a bi-wire setup the cable feeding the the higher frequncies no longer handles the large magnetic fields caused by the high current needed to produce bass. Bi-wiring does not affect the bass fundamentals, but the treble signal now travels a less disturbed path. It's a little like the differnce between swimming through waves versus through smooth water. Bass frequencies will sound better because bass definition is actually located in the midrange and higher frequencies. It is always wothwhile to take advantage of the benefits of bi-wiring when the speaker manufacturer has gone to the extra expense of providing this capability. The performance benefits of reducing distortion in this way are substantial."