The way sectors are numbered on a hard disk. Created with a low-level format, the optimum interleave is determined by the speed of the drive. In a 1:1 interleave, sectors are one after the other (0,1,2,3, etc.). A 2:1 interleave alternates them (0,4,1,5,2,6,3,7).
With a 1:1 interleave, the disk controller must be fast enough to read sector 2 after it reads sector 1, otherwise the beginning of sector 2 will pass by the read/write head and require a full rotation to come under the head again. A 2:1 or 3:1 interleave provides more time to read sequential sectors in one rotation. See sector