Back around the turn of the century when digital audio extraction (DAE) was relatively new, some drives couldn't provide audio data from precise locations. Each time it was told to read a block of audio, a drive might produce data that was shifted slightly ahead or slightly behind. When an adjacent block of data from a subsequent read is shifted, it will either begin with samples repeated from the previous block, or samples between the blocks will be omitted. In DAE, this phenomenon is often called jitter or synchronization error. To compensate for this problem, EAC is able to overlap every read in order to detect and correct any misalignment.

Essentially all drives produced today have a feature called 'Accurate Stream' which significantly reduces (if not completely eliminates) the chance that audio data will be shifted between successive reads. When informed that a drive has this feature, EAC will only periodically overlap its reads to check for synchronization problems.

From hydrogenaudio.