Actually a good question... And I found the answer from a link over on GeekNews
Yamaha announces the development of a new CD-R/RW laser pickup control technology that enables the "drawing" of characters and pictures on CD-R discs. This new technology will be demonstrated at the CeBIT (Mar. 2002) in Hanover, Germany.
Yamaha has utilized the CAV technology in all its CD-R/RW drives as Yamaha firmly believes that the CAV system is the most suitable writing control method for high speed recording. The CAV method allows the disc rotation speed to be kept constant while the velocity of pick up laser power is continuously adjusted, letting the disc to be written at both optimum speed and quality. Yamaha has been able to improve both the laser power and the disc rotation control capability significantly and that has allowed much further precision which affords the CD-RW recorder to write and draw characters and picture on the CD-R disc. This ability allows the user to write any type of "memo" such as "date", "contents", "logo", "pictures" etc. on the un-used area of the disk.