This was a protection that really frightened people for a while, as it was deemed uncopyable for a long time. In actuality, though the idea is quite clever, it is also quite easy to describe.
First, one must know how a drive seeks for a sector. When
the laser is positioned at sector 500, for example, and must seek to sector
100, it calculates how far backwards it must go from the current position
in order to get to sector 100. The drive isn't able to spin 100% accurately
to the correct point, so the drive will actually go a few sectors behind
sector 100, and seek forward until it sees in the synch and subchannels
of a sector a flag which says: "Hey, I'm sector 100!"
Tagès works by having multiple sectors
with the same sector number. The guard module first reads a sector before
any of the doubled sectors (whose real name is "twin sectors"), then one
in the first doubled range.
It saves this value. It then seeks to a sector before any of the twin sectors, then one after any of them, and then seeks to the same sector it seeked to before. It will go back to where it "should" be, but finds the second twin sector (which has different data in the user data area, the place where actual information is stored). It compares the the sectors, and if they are different, it assumes that the disc is an original. Since a copying program reads a disc in progressive order (1, 2, 3, 4, 5, ...), it will skip over the hidden range. So, when the Tagès guard module reads the twin sectors on a copy (which of course are not truly "twin"), it will get the same data, and know that the disc is a copy.
No programs are yet able to copy this protection, but be sure that some will appear soon. And of course, check the CD Freaks board for any breaking news or new tools.