Odd Sector Timing:

The layout of the sectors within a track has a critical effect on the speed at which the disk is read. The is most evident when a disk formatted with the older 810 is read and compared to a "fast formatted" (C ROM) disk. On a fast formatted disk the positions of the sectors are optimized for the most efficient reading. In any case, your drive can only format a disk with only one sector layout scheme, be it the fast or older, slower scheme. (All 810's shipped since December 1981 are fast format, anyway.)

What is done is that the software manufacturers arrange the sectors on the disk so they read in at some strange, uneven rate. The software has routines to time the reading of sectors, and to verify that the non-standard format is present.

One disadvantage to this technique is that the timing is critical. If the drive speed is a little off or a non-Atari drive is used the program won't boot. An advantage is that the SIO sounds may be disabled through the Atari OS. You won't hear the odd timing of the sector reads, and you won't even be aware that this type of copyguarding is being used.