The following is a quote from a lengthy list of FAQ's on the Van Den Hul Website:
"3-8 Q: What is the so-called “burning-in” of a cable ?
A: Any cable’s conductors consist of many groups of atoms, clustered in crystals or atom clusters. The borders between these crystals or clusters are critical zones where electron movements are hindered when the condition of these boundaries is getting worse. This worsening takes place e.g. when your loudspeaker cable is moved every day and/or chemical activity is around - latter amplified by higher humidity levels, elevated temperatures and air pollution. Also imperfections in the cable insulation can trigger conductivity changes.
Burning-in is the effect that, despite any affected crystal borders, the sound quality improves again thanks to electron movement through conducting zones like so-called electron tunnels. But... when you move a cable again, those tunnels are “broken” and you then have to start again by playing your music. You may wonder why (especially) on Sunday evening the music sounds so good again, this after a week with plenty of sound problems. Your cables finally are burned-in. But as soon as the killing Hoover (vacuum cleaning) hour on Monday morning comes and the over-active cleaning lady messes about with your cables, the problem returns: your cables are “burned-out” again, remaining that way until the following Sunday..."
